Getting the best mileage from the squeeze-bore guns?

[tomo pauk]

The almost 400m/s performance doesn't show up until late in the war.

It does not?

A substitute for the taper bore guns needs to be ready in 1940/early 41.

Covered in the post #42 here, at least for the 28/22.
No 42 mm type. The 47mm and French 75mm get the adapter.
Historical 75/55 is also never made, but the adapters for the 75mm ATG and tank guns are made, gets them to the Panther's level of AP performance with the appropriate ammo. Not for all and any such a gun, but certainly in the hundreds in 1942.

The IG 18's first HEAT shell was the one that had 260m/s velocity and had the 45mm at 30 degree performance. Used a powder charger of 71g.
It took a powder charge of 100g to get to 345m/s. Use of the longer barrel on the IG 37 may have made up the difference to 389ms (or higher) and there is no technical reason why a longer barrel could not have been used in 1940/41.

I've linked to the IG 37 article. That gun used same ammo that the Pz IVD or the -IIIN used, and not what the IG 18 used.

I am liking the Czech 4.7cm gun more and more.

You don't say

 

[Shortround6]

It does not?

Since the IG 37 doesn't seem to show up until 1944 I would appreciate any reference that says otherwise.
This just when it seems to have shown up, not that it could not have been done before.

I've linked to the IG 37 article. That gun used same ammo that the Pz IVD or the -IIIN used, and not what the IG 18 used.

The IG 37 sources can't seem to agree with themselves, let alone each other.

7.5 cm Infanteriegeschütz 37 - Wikipedia

en.wikipedia.org

says that the IG 37 used 75 x 89 mm R ammo while the gun in the PZ IV used 75×243mm R (?).

I have two of the books listed in sources of the Wiki article. The first book doesn't give the length of the cartridge case or the powder charge/s and only gives the MV of the standard HE shell. My copy of the book makes no mention of the HEAT round.
The 2nd source listed doesn't give details of specialty projectiles, just one velocity to cover everything, sometimes even different barrel lengths. But lists the short cartridge in the IG 37/42 and not the tank gun cartridge.
3rd source is the one I am using most of the time and I checked and I have the 2002 edition which is the one that is listed and I cannot find a 385ms velocity in the sections on the IG 18, IG L/13, IG 37 or IG 42.
However IG 37 was able to use a 100G charge to propel the standard 6kg HE shell at 280ms instead of the 210ms out of the IG 18. This may have been the charge (100g) that the IG used to get 345ms out of it's shorter barrel with the 3.05kg HEAT shell. Maybe it could have been done earlier if they had built the longer barrels and muzzle brakes earlier.
This book claims the IG 37 and IG 42 used the same ammo as the IG 18.
There are a couple of possible sources of confusion.
The Geb K 15 (old Czech mountain gun) fired a 4.5kg HEAT shell and perhaps there is some confusion.
The Geb G 36 (standard German mountain gun) fired a 4.4kg HEAT shell at 390m/s using charge 4 and not the max charge 5. In part due to recoil problems.

While trying to research this further I found the Soviet 76 mm regimental gun M1943 which is basically a short 75mm gun on a Soviet 45mm AT gun carriage and since the soviet 45mm AT gun carriage is pretty much the German 3.7cm AT carriage????
It is supposed to use a 76.2 × 167 mm R cartridge

at

76-mm-Regimentkanone M1943 – Wikipedia

de.wikipedia.org

I also found a trajectory chart for the earlier 76 mm regimental gun M1927 which used the same ammunition on this website.

76 mm HEAT Instructions

A blog about World War II era archive documents, primarily dealing with armoured warfare.

www.tankarchives.com

The trajectory column is giving the max high of the shell on the path to the target.

You can judge for yourself how practical some of the ranges were.

German Projectiles - 75mm Projectiles

German Projectiles H.E. Projectile Type 34, 75mm German Designation: Sprgr. Patr. KwK (34) (Spr...

ww2data.blogspot.com

has a cutaway and information of the HEAT round for the IG 18 but no performance figures for either penetration or velocity/range.
The design certainly looks like an early one. There was supposed to a 2nd design but penetration not much better.

Hope this helps.

 

[tomo pauk]

Since the IG 37 doesn't seem to show up until 1944 I would appreciate any reference that says otherwise.
This just when it seems to have shown up, not that it could not have been done before.

You were inquiring about the 37mm pak and what to do with it. The IG 37 was perfectly doable already pre-war.

The IG 37 sources can't seem to agree with themselves, let alone each other.

You are right. I was wrong saying that the IG 37 used the same ammo as the Pz-IVD did.

The towed gun that was using that ammo was the Pak 50, or basically the out-bored out-bored Pak 38 that also gotten a good part of it's barrel chopped off. That ammo/gun combination would've over-stressed the carriage of the Pak 36.

I have two of the books listed in sources of the Wiki article. The first book doesn't give the length of the cartridge case or the powder charge/s and only gives the MV of the standard HE shell. My copy of the book makes no mention of the HEAT round.
The 2nd source listed doesn't give details of specialty projectiles, just one velocity to cover everything, sometimes even different barrel lengths. But lists the short cartridge in the IG 37/42 and not the tank gun cartridge.
3rd source is the one I am using most of the time and I checked and I have the 2002 edition which is the one that is listed and I cannot find a 385ms velocity in the sections on the IG 18, IG L/13, IG 37 or IG 42.
However IG 37 was able to use a 100G charge to propel the standard 6kg HE shell at 280ms instead of the 210ms out of the IG 18. This may have been the charge (100g) that the IG used to get 345ms out of it's shorter barrel with the 3.05kg HEAT shell. Maybe it could have been done earlier if they had built the longer barrels and muzzle brakes earlier.
This book claims the IG 37 and IG 42 used the same ammo as the IG 18.

IG 18 ammo was fired with up to 70+g of propellant. The 100g propellant charge on the IG 37 will indeed require for a longer barrel to take advantage of that.

While trying to research this further I found the Soviet 76 mm regimental gun M1943 which is basically a short 75mm gun on a Soviet 45mm AT gun carriage and since the soviet 45mm AT gun carriage is pretty much the German 3.7cm AT carriage????
It is supposed to use a 76.2 × 167 mm R cartridge

One might see from where the Germans gotten the idea for the Pak 50...
Trying to see just how much the carriage of their light AT guns can handle would've paid good dividends to the Germans. Even if the gun from the Pz-IV does not fit conveniently on the carriage of the 37mm pak, it might've fit on the carriage of the Czech 47mm. So basically they might've gotten a good 47mm to weight just under 500 kg, and the short 75mm that can do close to 500 m/s with HEAT while weighting under 750 kg in the firing position. No exotic materials needed, and the reasonably capable guns are basically free, or they cost a fraction of the cost of a bespoke gun.

 

[tomo pauk]

Germans spent a pretty penny on the ammo for the 7.9mm anti-tank rifle, the thing being here that projectiles were mostly tungsten. About 510 thousand of rounds was present in their inventories already by May 1st of 1940. Even at just 10 grams a round (my assumption), that works to 5100 kg of tungsten. By June 1st of 1941, and despite the use in 1940, that balooned to more than 2.6 millions of rounds. Heer received in July of 1941 almost 740 000 these rounds.
They could've just go with a 13mm AT rifle (say, spin-off from the ww1 stuff) instead of the Pzb 38 & 39, use hardened steel ammo and get to the same penetration values; stick a good muzzle brake on it and there it is. Saves them tons of tungsten.

The 28/22 ammo count went to almost 350 thousand by June 1st '41; July saw more than 170 000 of that ammo received. I'm not sure about the HE/APCR ratio.

 

[Shortround6]

Germans spent a pretty penny on the ammo for the 7.9mm anti-tank rifle, the thing being here that projectiles were mostly tungsten. About 510 thousand of rounds was present in their inventories already by May 1st of 1940. Even at just 10 grams a round (my assumption), that works to 5100 kg of tungsten. By June 1st of 1941, and despite the use in 1940, that balooned to more than 2.6 millions of rounds. Heer received in July of 1941 almost 740 000 these rounds.
They could've just go with a 13mm AT rifle (say, spin-off from the ww1 stuff) instead of the Pzb 38 & 39, use hardened steel ammo and get to the same penetration values; stick a good muzzle brake on it and there it is. Saves them tons of tungsten.

Quite true. The Core for the 75/55 ammos was just about 2lbs so roughly 900 times what your estimate of the 7.9mm core? Not going to argue about a few percent or even 10%.
The 7.9mm AT rifles was one of those expensive toys. The early non Tungsten core round was the one that had the tear capsule inside. Difficulty of manufacture was????
effect in combat was zero. None of the troops that survived getting hit by it in the vehicles ever reported any effects.
The PzB 38 heavy and overly complicated for the intended use.

When fired the barrel moved for about 9cm and then Breech block unlocked and dropped, the cartridge case was automatically ejected and the Breech block held open. Gunner (or assistant) feed a new round into the breech, Gunner release a catch and the breech rose while the barrel returned to firing position. All very clever but expensive to make and prone jamming when dirty. Replaced by the PzB 39 where the gunner pulled on the pistol grip to to drop the breech block to eject and reload. Saved about 3.5kg and lot of machining.

The 28/22 ammo count went to almost 350 thousand by June 1st '41; July saw more than 170 000 of that ammo received. I'm not sure about the HE/APCR ratio.

You will get little argument. While the special troops may have wanted a special AT gun (rationale) I don't think the 28/20 really delivered. And the HE round was an evil joke. Delivering 5g of HE it makes a single shot 20mm look good and with each HE round fired the very expensive barrel is getting nearer to the end of it's life.
Give the special troops some Solothurn S18-1000 guns and call it a day and save the Tungsten.

 

[spiralcopter]

Squeezebore guns are basically an alternative way of accomplishing the same thing as as sabot; that is, they reduce the sectional density of the projectile relative to the swept volume of the barrel bore in order to improve velocity while maintaining acceptable intermediate projectile ballistics.

A round like APCR works by having an armor piercing core wrapped by light alloy so the projectile sectional density relative to the bore diameter is low. The light alloy wrapping creates a lot of drag, but over short ranges this is acceptable.

A sabot works by having this light alloy "wrap" fall off, essentially, while a squeezebore works by filling up the space between the projectile and the bore with a skirt that gets smashed down to reduce frontal area and so preserve the aerodynamics.



Given all of that, the first question to ask yourself if you are designing a squeezebore is why not use a sabot instead?

 

[z42]

Given all of that, the first question to ask yourself if you are designing a squeezebore is why not use a sabot instead?

Easy to say in retrospect that squeeze bore was a failure. But it took a lot of work and time to make sabots work acceptably, so perhaps not that obvious at the time.

Then again, the issues with the squeeze bore concept are to some extent more fundamental than merely making the sabots fall off smoothly, which even at the time could have given a hint that one set of issues might be solved with application of more elbow grease and the other not.

 

[Shortround6]

Squeezebore guns are basically an alternative way of accomplishing the same thing as as sabot; that is, they reduce the sectional density of the projectile relative to the swept volume of the barrel bore in order to improve velocity while maintaining acceptable intermediate projectile ballistics.

A round like APCR works by having an armor piercing core wrapped by light alloy so the projectile sectional density relative to the bore diameter is low. The light alloy wrapping creates a lot of drag, but over short ranges this is acceptable.

A sabot works by having this light alloy "wrap" fall off, essentially, while a squeezebore works by filling up the space between the projectile and the bore with a skirt that gets smashed down to reduce frontal area and so preserve the aerodynamics.



Given all of that, the first question to ask yourself if you are designing a squeezebore is why not use a sabot instead?

The squeeze bore was an attempt to keep the internal pressure in the barrel high for as long as possible. A light projectile has quicker acceleration and less barrel time than a heavier projectile. Peak barrel pressure was often achieved (wither they wanted to or not) in the first few (3-6?) calibers of travel of the projectile. Like in a 50cal length AT barrel peak pressure was hit at around the time the projectile hit the 6 caliber mark, as the projectile continued to move forward the expanding volume of the barrel meant a decreasing pressure in the barrel despite the propelling charge still burning. German 75mm AT gun has the Projectile moving about 450mm at the 6 caliber mark with a marked increase in "chamber" volume.
The expanding gas is still pushing on the base the of the projectile but at the muzzle it can be around 1/6-1/8 of the peak pressure.
Due to the smaller volume inside the barrel of the taper bore gun for the same theoretical powder charge and peak pressure the pressure inside the barrel will stay higher and so exert more force before the projectile leaves the barrel.
The real question is if the greatly increased barrel wear and the difficulty of making taper barrel guns is worth the result.
Germans were jumping through hoops to try to get around these two problems with the 75/55gun. They went to a 3 piece barrel in which only the 1st piece (closest to the Breech) was rifled and had a constant barrel bore. 2nd piece was smooth bore (not rifled) and had the taper. 3rd piece was not tapered and was also smooth bore. These extra pieces could replaced as they wore to keep up 'performance'. Easier than replacing the entire barrel but not as easy to make as a conventional gun. One also wonders if the smooth bore sections affected accuracy?

 

[Juha3]

There is level ground in Switzerland? Where is the photo of him riding uphill pulling the gun?

I have a fair amount of cycling experience, including from the military, and I would say the real problem is the downhills. It's not fun to come into a sharp bend with a heavy load on the end of the draw bar while cycling downhill.

And Switzerland has flat areas in the valleys

 

[tomo pauk]

Germans were jumping through hoops to try to get around these two problems with the 75/55gun. They went to a 3 piece barrel in which only the 1st piece (closest to the Breech) was rifled and had a constant barrel bore. 2nd piece was smooth bore (not rifled) and had the taper. 3rd piece was not tapered and was also smooth bore. These extra pieces could replaced as they wore to keep up 'performance'. Easier than replacing the entire barrel but not as easy to make as a conventional gun. One also wonders if the smooth bore sections affected accuracy?

The 75/55 mm was with the cylindrical barrel for the most part; the last meter or so of the barrel was tapering down to 55mm. That conical, last part was attached to the rest of the barrel.

 

[spiralcopter]

Easy to say in retrospect that squeeze bore was a failure. But it took a lot of work and time to make sabots work acceptably, so perhaps not that obvious at the time.

Then again, the issues with the squeeze bore concept are to some extent more fundamental than merely making the sabots fall off smoothly, which even at the time could have given a hint that one set of issues might be solved with application of more elbow grease and the other not.

Correct! Sabots were a very new technology in the WW2 timeframe, and there were significant technical challenges that were unsolved/partially solved (see the accuracy problems with early British 17 pounder APDS).

One interesting advantage of the squeezebore to consider is that there simply is not a sabot. Usually sabots are not a problem, or at least once the tricky engineering of getting them to cleanly separate is solved. But there are sometimes when having big, fast-moving chunks of aluminum is a bother. One of the latest WW2 German squeezebore projects was a high-altitude flak gun, the reasoning apparently being that they didn't want sabot petals raining down on their own cities.


Similarly, squeezebores were briefly investigated by the US Air Force as a potential option for a high-velocity aircraft weapon in... I wanna say the 60s-ish. Sucking up sabots is *very* hard on jet engines, so the squeezebore was seen as a potential alternative way forward. They never got the collapse of the projectile skirt to be consistently concentric enough to produce good accuracy, so the project was dropped.

The squeeze bore was an attempt to keep the internal pressure in the barrel high for as long as possible. A light projectile has quicker acceleration and less barrel time than a heavier projectile. Peak barrel pressure was often achieved (wither they wanted to or not) in the first few (3-6?) calibers of travel of the projectile. Like in a 50cal length AT barrel peak pressure was hit at around the time the projectile hit the 6 caliber mark, as the projectile continued to move forward the expanding volume of the barrel meant a decreasing pressure in the barrel despite the propelling charge still burning. German 75mm AT gun has the Projectile moving about 450mm at the 6 caliber mark with a marked increase in "chamber" volume.
The expanding gas is still pushing on the base the of the projectile but at the muzzle it can be around 1/6-1/8 of the peak pressure.
Due to the smaller volume inside the barrel of the taper bore gun for the same theoretical powder charge and peak pressure the pressure inside the barrel will stay higher and so exert more force before the projectile leaves the barrel.
The real question is if the greatly increased barrel wear and the difficulty of making taper barrel guns is worth the result.
Germans were jumping through hoops to try to get around these two problems with the 75/55gun. They went to a 3 piece barrel in which only the 1st piece (closest to the Breech) was rifled and had a constant barrel bore. 2nd piece was smooth bore (not rifled) and had the taper. 3rd piece was not tapered and was also smooth bore. These extra pieces could replaced as they wore to keep up 'performance'. Easier than replacing the entire barrel but not as easy to make as a conventional gun. One also wonders if the smooth bore sections affected accuracy?

Sorry, but your physics are simply wrong here. The force acting on the base of the projectile is equal to pressure times cross-sectional area (do the math, the dimensional analysis checks out). Increasing the pressure by tapering the bore diameter does nothing, as any increase in pressure is counteracted by the reduction in cross-sectional area.

 

[Shortround6]

Sorry, but your physics are simply wrong here. The force acting on the base of the projectile is equal to pressure times cross-sectional area (do the math, the dimensional analysis checks out). Increasing the pressure by tapering the bore diameter does nothing, as any increase in pressure is counteracted by the reduction in cross-sectional area.

The physics is rather complicated and a bit beyond me as we have to know the pressure in curve in the barrel and the pressure curve is not constant.
We have to know where in barrel the projectile is when the powder stops burning (generating additional pressure) and that is further out in the barrel than peak pressure.

a bit simplistic but getting a better one is time consuming.
A very light projectile accelerates quickly and pressure curve drop quicker than with a heavy projectile.
Smokeless powder burns at different rate/s depending on the pressure it is burning under. A very light projectile can accelerate and get out of the barrel with some powder left unburned in the barrel and blown out the muzzle. You can get a bit unburned even with a heavy projectile but the quantity is less.

Force times area times weight does govern the speed of the projectile but the force is not constant and the force (pressure) is not same at different points in the projectile's travel inside the barrel.
Now most (all?) taper bore guns did not use constant taper bores which really screws things up. British 2pdr stuck a taper bored section on the end of the barrel.

And they found that while velocity and penetration was reduced with the adaptor removed it did not change penetration by very much and the armored car crews were quite happy to leave it off so they could use different types of ammo without having to climb outside the vehicle and unscrew the adaptor and screw it back on. Peak performance took a back seat to getting shot.
One is also entitled to wonder how much of an improvement a non-tapered muzzle device would have made? only 5-10m/s or more?
Each German gun used a different arrangement straight bore and tapered bore section/s. I believe the 28/20 used two different taper bore sections?
Without knowing the volume of the bore in each section of barrel (or the dimensions of the taper) figuring out the pressure (or even trying to do and educated guess) is beyond me.
Considering that changing the powder will change the powder burn rate at different pressure levels also really screws things up.

As it seems to have turned out the taper bore principle may have been real, just not really as useful as thought and the problems were large.

 

[spiralcopter]

The physics is rather complicated and a bit beyond me as we have to know the pressure in curve in the barrel and the pressure curve is not constant.

The physics are not beyond me, and I will explain them.


Guns are piston engines. The barrel is the cylinder and the bullet is the piston. They're somewhat unusual among piston engines in that the piston does not reciprocate; it just undergoes one stroke, which is the power stroke, and then the piston goes on its merry way into whatever happens to be unlucky enough to lie in its path.

(Ballistics textbooks will say this verbatim too)

The force acting on the bullet at any point in its trip down the barrel is the cross-sectional area of the bore (so half the caliber squared times pi) times the pressure acting at the base of the bullet (this distinction is significant. Bullets move fast enough down the barrel that the system significantly violates Pascal's Law).

The total work performed on the bullet during its time in the barrel is therefore the integral of pressure with respect to bullet travel down the barrel times cross sectional area. Do the algebra and this checks out in terms of dimensional identites. If you squint, this math looks similar to the equation for gross horsepower in a piston engine of BMEP x displacement x RPM and that's not a coincidence. Guns are piston engines.

The squeeze section of a squeezebore does nothing whatsoever to accelerate the projectile or make better utilization of the barrel length. Quite the opposite, and the math I described in the previous paragraphs explains exactly why. Reducing cross-sectional area to increase pressure will only increase pressure by the same amount that the cross-sectional area is being reduced by (actually a bit less due to gas flow reasons), and force acting on the projectile, which is what we actually want to maximize, is the product of area and pressure. Trading one off for the other does literal actual nothing.

If the squeeze section somehow increased the performance of the gun, you would expect to see the trend be towards guns where the tapering section is as long as possible, but in fact the opposite was the case. The Germans were moving towards guns where the tapered section of the bore was as short as possible, because aside from being easier to make, they found that it reduced the wear problem as well.

The squeeze section of a squeezebore is just to smash the skirt of the projectile into an aerodynamic configuration. The skirt is essentially the same thing as a sabot, except where a sabot falls off the projectile, the skirt gets mashed flush against the projectile body by the squeeze section.

 

[Shortround6]

Propellent gas production in a gun barrel is rathe complicated. More so than a gasoline engine.

As an example, the original 30-06 military cartridge fired a 150grain bullet at 2700fps using a 50 grain charge of powder and a peak pressure of 52,000lb per sq in.
in the late 30s they came up with the M2 load which used a 150grain bullet at 2800fps at a much lower peak pressure using a different powder. The new powder gave a higher average pressure in the barrel.
Smokeless powder does not have a constant burn rate. It's burn rate varies depending on the pressure it is burning under. This is why changing bullet weight can really affect peak pressure as can small changes in powder load at near max loads.

Because of these different burning rates you can get very different velocities at the same peak pressures using different powders.

Smokeless powder not only has different exact formulations, even the same formulation burns differently depending on grain size and configuration (long skinny, short fat. One or more tubes running through the grain) and the use of deterrent coating/s that slow down initial flame spread.
Very large powder charges and light projectiles can also mean incomplete combustion with considerable quantities of powder (fuel) burning after the projectile has left the barrel.
Think supercharged engine with flames belching from the exhaust pipes/stacks.
The high velocity guns were not very efficient, even with longer barrels they were not getting proportionately higher velocities for amount of powder used even when figuring out kinetic energy.

Taper bore guns were an attempt to make a more efficient gun. Results without technical information from test facilities is guess work. The German guns were all different inside so we don't get any information from scale effects (if any). The other German guns are so different is size/weight of projectiles that little can be learned there. German 75/55 used both a lighter projectile and a smaller power charge than the Pak 40 7,5cm so things are pretty much guess work. Type of powder may not be any help as while we know the type we may not know grain size, shape or perforations (if any) or any deterrent coatings and even knowing all of that, trying to figure out the burn rate in the barrel (pressure curve) is guess work.
British 2pdr is also short of information. Somebody had to fired the thing both with and without the device attached to the barrel to see if did give more velocity.
The squeezed down projectile would have better down range ballistics even if going a bit slower to start. But published information is lacking.
Problems with the 2pdr vs the Germans is that the 2pdr Little John lengthened the barrel which should have increased velocity somewhat on it's own.
This is countered by the ports in the barrel in/before the tapered section which would lower the pressure in the barrel by an unknown amount. Lots of ports of unknown (to us) size.

A decent test program would have drilled a barrel out and fitted some sort of pressure sensors along the barrel to find out what was going on. This was not new. John A. Dahlgren and done this for the US before US civil war which led to both the famous Dahlgren guns and their distinctive shape and a slower burning black powder for large artillery to lower peak pressures.
I would doubt that the Germans and British didn't do some sort of test program. The gun makers didn't forget Dahlgren and resort to guess work with smokeless powder.

 

[spiralcopter]

I don't know why you're arguing this so vociferously when, by your own admission, you don't grasp the physics. With respect, you are straightforwardly wrong about this and the reason why you are wrong is internal ballistics 101.

The idea that the squeeze section somehow improves efficiency is clear nonsense. Look up the dimensional identities of the physical terms you are using. Force is what accelerates the bullet. Pressure is not the same thing as force. Pressure is force divided by area, which is why it is in units like pounds per square inch, or Pascals, which are Newtons per square meter. Tapering the bore section of a working cylinder will not increase the force tangential to its long axis; any increase in pressure that results is because of a proportional decrease in area. This idea very obviously does not work.

Gas generation rates of propellants are indeed complicated, and also completely irrelevant to this question. The work performed on a projectile in a barrel is the integral of the pressure curve with respect to the distance travelled times the cross sectional area. That equation does not change. It doesn't matter what shape that pressure curve takes, in much the same as it doesn't matter whether an engine uses three or four valves per cylinder or what the shape of the piston head is; the gross horsepower formula is still BMEP x RPM x displacement.

There is no evidence whatsoever that Gerlich, who invented the taperbore, thought that his system improved internal ballistic efficiency. It simply is not mentioned in the original patent at all. But do you know what is mentioned? Squeezing down the flanges on the projectile in order to reduce air resistance. Because that's what the purpose of a squeezebore is.

Don't believe the guy who invented it? Read this US Air Force Autocannon Ammunition Development Summary and see what it has to say about squeezebores. It makes it very clear that the point of a squeezebore is to have low projectile sectional density in the full-caliber section for maximum in-bore acceleration, but a high ballistic coefficient once the skirts are folded flush with the projectile body.

I have one or two other archival documents on the topic which are debriefs of German scientists, and I don't know if these are publicly available. Suffice it to say, every single one of them is consistent with the idea that squeezebores are a way to combine a streamlined projectile with a high bore swept volume, and there is no indication that anyone associated with the development of the same ever believed otherwise.

I don't know where your notion on this came from, but it is contradicted by elementary internal ballistics science and a wealth of archival documents.

 

[yulzari]

I don't know why you're arguing this so vociferously when, by your own admission, you don't grasp the physics. With respect, you are straightforwardly wrong about this and the reason why you are wrong is internal ballistics 101.

The idea that the squeeze section somehow improves efficiency is clear nonsense. Look up the dimensional identities of the physical terms you are using. Force is what accelerates the bullet. Pressure is not the same thing as force. Pressure is force divided by area, which is why it is in units like pounds per square inch, or Pascals, which are Newtons per square meter. Tapering the bore section of a working cylinder will not increase the force tangential to its long axis; any increase in pressure that results is because of a proportional decrease in area. This idea very obviously does not work.

Gas generation rates of propellants are indeed complicated, and also completely irrelevant to this question. The work performed on a projectile in a barrel is the integral of the pressure curve with respect to the distance travelled times the cross sectional area. That equation does not change. It doesn't matter what shape that pressure curve takes, in much the same as it doesn't matter whether an engine uses three or four valves per cylinder or what the shape of the piston head is; the gross horsepower formula is still BMEP x RPM x displacement.

There is no evidence whatsoever that Gerlich, who invented the taperbore, thought that his system improved internal ballistic efficiency. It simply is not mentioned in the original patent at all. But do you know what is mentioned? Squeezing down the flanges on the projectile in order to reduce air resistance. Because that's what the purpose of a squeezebore is.

Don't believe the guy who invented it? Read this US Air Force Autocannon Ammunition Development Summary and see what it has to say about squeezebores. It makes it very clear that the point of a squeezebore is to have low projectile sectional density in the full-caliber section for maximum in-bore acceleration, but a high ballistic coefficient once the skirts are folded flush with the projectile body.

I have one or two other archival documents on the topic which are debriefs of German scientists, and I don't know if these are publicly available. Suffice it to say, every single one of them is consistent with the idea that squeezebores are a way to combine a streamlined projectile with a high bore swept volume, and there is no indication that anyone associated with the development of the same ever believed otherwise.

I don't know where your notion on this came from, but it is contradicted by elementary internal ballistics science and a wealth of archival documents.

I am reminded of the definition by the late great Keith Duckworth that 'the power of an engine is the size of the bang times the number of bangs per minute'.

 

[spiralcopter]

I am reminded of the definition by the late great Keith Duckworth that 'the power of an engine is the size of the bang times the number of bangs per minute'.

"Ain't no replacement for displacement!"

Another term that comes up a lot is "swept volume." In a world where the propellant gas expands adiabatically, and where all the cows are spherical and frictionless, increasing the caliber of the gun can be thought of as equivalent to increasing the length of the barrel. Both are ways of increasing the swept volume and squeezing more projectile KE out of a given amount of propellant CE. But increasing caliber doesn't usually make velocity go up, because scaling up the projectile increases its mass by a cubic function of the caliber scaling factor.

The only way to come out ahead is to make the projectile very very lightweight relative to caliber, but that compromises aerodynamics.

Which leads inexorably to the logic of discarding sabots and squeezebores.

 

[monicac]

Propellent gas production in a gun barrel is rathe complicated. More so than a gasoline engine.

As an example, the original 30-06 military cartridge fired a 150grain bullet at 2700fps using a 50 grain charge of powder and a peak pressure of 52,000lb per sq in.
in the late 30s they came up with the M2 load which used a 150grain bullet at 2800fps at a much lower peak pressure using a different powder. The new powder gave a higher average pressure in the barrel.
Smokeless powder does not have a constant burn rate. It's burn rate varies depending on the pressure it is burning under. This is why changing bullet weight can really affect peak pressure as can small changes in powder load at near max loads.

Because of these different burning rates you can get very different velocities at the same peak pressures using different powders.

Smokeless powder not only has different exact formulations, even the same formulation burns differently depending on grain size and configuration (long skinny, short fat. One or more tubes running through the grain) and the use of deterrent coating/s that slow down initial flame spread.
Very large powder charges and light projectiles can also mean incomplete combustion with considerable quantities of powder (fuel) burning after the projectile has left the barrel.
Think supercharged engine with flames belching from the exhaust pipes/stacks.
The high velocity guns were not very efficient, even with longer barrels they were not getting proportionately higher velocities for amount of powder used even when figuring out kinetic energy.

Taper bore guns were an attempt to make a more efficient gun. Results without technical information from test facilities is guess work. The German guns were all different inside so we don't get any information from scale effects (if any). The other German guns are so different is size/weight of projectiles that little can be learned there. German 75/55 used both a lighter projectile and a smaller power charge than the Pak 40 7,5cm so things are pretty much guess work. Type of powder may not be any help as while we know the type we may not know grain size, shape or perforations (if any) or any deterrent coatings and even knowing all of that, trying to figure out the burn rate in the barrel (pressure curve) is guess work.
British 2pdr is also short of information. Somebody had to fired the thing both with and without the device attached to the barrel to see if did give more velocity.
The squeezed down projectile would have better down range ballistics even if going a bit slower to start. But published information is lacking.
Problems with the 2pdr vs the Germans is that the 2pdr Little John lengthened the barrel which should have increased velocity somewhat on it's own.
This is countered by the ports in the barrel in/before the tapered section which would lower the pressure in the barrel by an unknown amount. Lots of ports of unknown (to us) size.
A decent test program would have drilled a barrel out and fitted some sort of pressure sensors along the barrel to find out what was going on. This was not new. John A. Dahlgren and done this for the US before US civil war which led to both the famous Dahlgren guns and their distinctive shape and a slower burning black powder for large artillery to lower peak pressures.
I would doubt that the Germans and British didn't do some sort of test program. The gun makers didn't forget Dahlgren and resort to guess work with smokeless powder.

Your point about smokeless powder burning differently under pressure is why I stopped trusting simple load tables and started watching pressure curves because the M2 load proves you can gain velocity while dropping peak pressure just by changing grain geometry and deterrent coating. That same principle shows up when you tune a pcp air rifle and realize the hammer strike and valve dwell shape the shot string far more than fill pressure alone. I have spent evenings watching how a heavier pellet shifts the whole curve and learned that internal ballistics is really about time and area under the curve rather than one big number. The taper bore experiments are fascinating because without port diameter and pressure tap data you are guessing and that is why Dahlgren drilling barrels and fitting sensors still matters. I would love to see a modern pressure trace on a Little John barrel because those ports before the taper bleed energy the longer barrel then tries to recover and only measurement settles it.