UK goes all-in on a HMG class gun in the mid-30'ies

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Well, they weren't "sniping".
152-160 rounds per second increases the chances of golden BBs over 50 or so projectiles a second even if they are bigger.

Well, that was hyperbole, in case it wasn't clear.;)
The point remains, fighter armaments were designed around estimates of how many hits it would, on average, take to shoot down an enemy plane. And on average, it took quite a few rifle caliber MG hits to shoot down a plane, hence why they needed a sizeable battery of fast firing guns.
An argument like ".303 was good enough since a hit to the head of the pilot wins the engagement", while superficially true, doesn't explain why LMG's were considered deficient and replaced and/or supplemented as soon as heavier guns were in a usable state. Particularly considering that heavier guns have a smaller probability of a headshot due to fewer guns and lower RoF.
 
If you read the report from the previous page the recommendation was two cannons and four .50 BMG's over four cannons or two cannons and four .303's.
To quote from that discussion linked on the previous page where Air Marshal Douglas wrote the Air Ministry:
Instead I recommend that armament of this aircraft should be standardised at 2 x 20 mm. and 4 x .5 guns, with ammunition for 15 secs. fire for all guns. If this requirement cannot be met, I am prepared to accept 2 x 20 mm. and 2 x .5 guns, with the same ammunition load as above.

While I don't have anything like encyclopedic knowledge of this topic, but perhaps the reason why I, nor the poster you answered to, aren't aware of Spitfires with 2x20mm + 4x.50 armament, is that this requirement could indeed not be met, which is why 2x20mm + 2x.5 instead became the standard armament?
 
An argument like ".303 was good enough since a hit to the head of the pilot wins the engagement", while superficially true, doesn't explain why LMG's were considered deficient and replaced and/or supplemented as soon as heavier guns were in a usable state.
Col Neel Kearby who was the leading P47 ace in the Pacific was shot down and killed by a Ki-43 armed with a single Vickers .303 and 12.7mm Breda.
 
While I don't have anything like encyclopedic knowledge of this topic, but perhaps the reason why I, nor the poster you answered to, aren't aware of Spitfires with 2x20mm + 4x.50 armament, is that this requirement could indeed not be met, which is why 2x20mm + 2x.5 instead became the standard armament?
My reply was in regards to the weight penalty of four cannons verse's two cannons and four .50's, the standard Spit armament was two cannon's four .303's and later after gyro sights two cannons two .50's.
 
IMO, it sort of depends on what you want. I did some play around with weights of both ammo and guns, and came up with the following:

Approx 300 rpg for 6x.50 AN/M2 Brownings and 150 rpg for 4x20mm Hispano Mk V weighs pretty much the same (320 kg or about 700 lbs).

6x.50s with 400 rpg is heavier than 4x20mm Mk V with 200 rpg

I didn't do a mix of 20mm or .50s, but if you got by the into about the weapons and weight per 100 rounds of ammo, you can probably figure it out with a calculator. I did my figuring on what a typical P-51D/H Mustang carried (1880 divided by 6 equals 313 rpg, so I rounded down to 300).

It also depends on if you want more firing time, or more rounds per burst. Going by the Mustang again (D/H models), with 4 .50s you got 400 rounds a gun for 30 seconds of firing time. With 6 you got 400 inboard pair, 270 for the rest, which means about 30/20 seconds, but more bullets on target while the outboard guns have ammo.
 
Also, IMO, we do have to consider what is sufficient ammo. Again, take the Mustang (D/H models), 2x400 and 4x270 rpg doesn't sound like a lot, but if you average it out, you get at least 300 rpg (313 is the actual number). The F-82 could carry 413 rpg per the XP-82 restoration (400 in the ammo boxes, 13 in the feed chutes it seems). If that also applies to the P-51, you might get an extra 10+ rpg in the feed chutes. Granted, the F-82 being a heavy fighter/twin engine aircraft that weighed a bit less all up and normal TO than two P-51H Mustangs and having two engines of the same power as the H (for the Merlin powered versions), you do have a bit of a power reserve and weight to play around with (though not a ton).

For reference, the F-80 carried 300 rpg in it's ammo boxes for it's 6 .50s, and when the Grumman F8F Bearcat carried 4 .50s, it used 300 rpg (down .2 .50s and 100 rpg compared to the F4U Corsair and the F6F Hellcat)., but when it was upgunned to 4 20mm cannons, it carried 205 rpg.

Let's look at a hypothetical, looking at the P-51D/H as the basis (and NAA looked at this for the Australian P-51s per drgondog, one of our resident Mustang experts and authors). Let's replace 2 of the .50s with a 20mm Hispano Mk 2 or even Mk 5. 20x110mm Hispano per round weighs about twice as much as .50 BMG per round. And .50 BMG takes up about 2/3ds the space that 20mm does in an ammo box. If the Bearcat example is accurate, if you keep 400 rpg for the .50s, for the 20mms you can run either 200 rpg for the same weight or up to 300 rpg (probably less is a bit more sensible) in the same space.

Of course, if you keep 270 rpg for the .50s, the math formula is the same. Again, it depends on what you see as sufficient.

The XP-51F/Gs that were armed it also seems again per drgondog normally carried 250 or so rpg (planed alternative armament was, per same source, 4x20mm cannons with 125 rpg), though I've seen several other sources mention up to 440 rpg. Also, looking at the info sheet on a P-51D's ammo door, the outboard guns when carrying 4 .50s could run up to 500 rpg for the outboard pair.

And of course, there's the deal of what do you wanna shoot down? Is your plane mostly going to be use fighter vs fighter, or as a bomber/recon interceptor? The latter would usually favor cannon armament, but P-51s armed with .50s had little trouble dispatching German and Japanese heavy fighters (some of which were the size of--or sometimes converted from--medium bombers), and had no trouble sinking the BV238 flying boat by strafing it. And the BV238 was the largest plane the Germans managed to build in World War II.
 
In fact BV 238 V1 was not destroyed by P-51s. According the Shores' and Thomas' 2nd TAF Vol 4 [p. 738] the a/c Drew destroyed was the 6-engined Potez-CAMS 161 proto. Bv 238 V1 was considerably damaged by Typhoons / 439 Sqn on 24 Apr 1945. It was scrapped in 1947 by Royal Engineer sappers. On thinner ice, IIRC I read somewhere that Germans let it sink because all knew that the war would end very soon and they though that the plane had better chances to survive as sunk in a lake than if they try to salvage it immediately and by doing so attracted more air attacks on the site. After the war Germans wanted to rise the plane and a RAF engineering officer visited the site and agreed that the plane was salvageable but the higher British authorities forbade that.
 
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Sorry to resurrect this thread, but playing around with more calculations, and we know that the USAAF and USN went with the .50 AN/M2 (later AN/M3) Browning MGs because they worked and did more or less what was asked of them at the time.

I actually propose that say a 4x20mm Hispano armed P-47 or P-51 or P-38 could've been a bit of a boon as far as weight savings. Namely that 6x.50 Brownings (about 65 lbs a gun) and either 400 (30 lbs per 100 rounds) or 300 rounds weighed more than 4x20mm Hispano Mk Vs or M3s or M24s (about 90 lbs a gun) and 200 (62 lbs per 100 rounds) and 150 rounds respectively. In fact, 4x20mm with 200 rpg is comparable in weight (if not a bit lighter) than 6x.50s with 300 rpg.
 
Unfortunately the weights for the 20mm guns are not strictly accurate or rather they are strictly accurate for a bare gun, UNINSTALLED.

Hurricane with belt feeds detached from the guns.
urricaneMkIIC.jpg.f723bba04ec58484187541934416930e.jpg

HispanoSectionedFront.jpg


The thing about the Hispano gun was that it could be feed by either the drum (60 rounds) or belt with the addition of a belt feeder to the basic gun.
There were several belt feeds but most (all?) of the guns could take the different models. Remove the feeder and a standard drum would mate up to the gun.

There were often a few adjustments that had to be done. Drum feed guns usually had a muzzle brake because the drum didn't use the extra recoil energy to operate the drum.
The belt feed guns uses the recoil to recock/tension the BFM spring.
Most of the drives weighed very close to what an empty drum weighed. A Hispano drum weighed about 10.8KG and the belt feeds went about 8kg if I remember right.

Installed weights can be affected by charging devices and heating arrangements.
The bigger guns also usually need bigger mounts/brackets.


Not a deal breaker but not quite the weight savings you may think.

The weight of the feeder is worth about 30 round of ammo not including belt links.
 
This is where I got the data from the M24 cannon (basically an updated M3):


And yes, the feeder will add about 15 lbs to the gun, though if you don't count the mounting cradle you drop almost 10 lbs. The British adopted a much more compact and lighter belt feeder (I think developed by Martin Baker), but that was post war and I don't think the USAAF/USAF and USN never adopted a similar feeder.

Even with that revision, 4x20mm with 200 rpg is still a savings of at least a couple 100 lbs over 6x.50s with 400 rpg.

However, if you cut the .50 ammo down to 300 rpg, you get basically the same weight as 4x20mm with 200 rpg.

You do have to look at things like ammo weight, possible damage, and desired firing time. As well as overall weight of the aircraft and engine power.

In short, if you want that type of gun and ammo load out in an interceptor fighter capable of 400+mph, and you're looking at Spitfire type range, you're going to want at least a 1500 hp engine, preferably more. The British didn't really get that from an inline V12 until the Merlin 60 series, or the early single stage Griffons, or tinkering with the boost limiters for the Allison V-1710s they got in Mustang Is and IIs. The Sabre had more power, but was heavier and not insanely reliable until about 1944, and the Hercules wasn't really in demand for fighters aside from the Beaufighter.

I'm not sure it was practical to carry even 6 .50s with 300 rpg or 4 20mm (Hispano type, even with the lightened late war/post war cannons) with 150 rpg in the Battle of Britain. You could go with the MGFF type cannons, but you'd either have to trade range/accuracy for fire rate or range/velocity/accuracy for a slower rate or fire with early war guns. Not to mention that most Oerlikon cannons were drum fed.
 
I'm not sure it was practical to carry even 6 .50s with 300 rpg or 4 20mm (Hispano type, even with the lightened late war/post war cannons) with 150 rpg in the Battle of Britain. You could go with the MGFF type cannons, but you'd either have to trade range/accuracy for fire rate or range/velocity/accuracy for a slower rate or fire with early war guns. Not to mention that most Oerlikon cannons were drum fed.
Yeah, not practical. There are some calculations earlier in this thread, but basically a Spit/Hurricane Mk I had 8 .303 guns each clocking in at about 10kg (+ ammo). So your budget is around 80 kg for the entire battery (+ ammo). So if the Hispano had been available at the time, reliable, and with the belt feed system, two Hispanos would have been an option, but you have to get rid off all the 303's. IMHO that would have been a more powerful armament than the historical, but again, not available at the time.

As for belt vs drum, interesting to note that contemporary aircraft guns have largely gone back to drums. Admittedly pretty different kinds of drums than the WWII variety, but still. Turns out that a 6000rpm Gatling will just rip belt links apart, or deform them sufficiently to easily cause a jam. So contemporary practice is so-called linkless feed systems, basically an electrically powered conveyor belt transporting the shells to the gun. Further, ejecting cases and links from a supersonic aircraft without damaging it isn't easy, so they tend to feed the spent cases back into the magazine.
 
I wanted to post this earlier, but couldn't remember where I found it previously. But the Royal Armouries in Leeds, England has a Hispano-Suiza Mk V cannon with a flat Martin-Baker flat feed mechanism. RA claims that including the feeder this cannon weighs only about 85 lbs (minus mounting cradle or similar fittings):

 
It's all well and good to say this is better than that but reality is very different. The .50 BMG was neither reliable or light weight, same for it's ammunition, the Hispano was out purely on reliability, the FF/M was light but it's feed wasn't reliable requiring 55 rounds instead of 60 to be loaded, it's ammunition likewise either worked or didn't, the various 12-13mm HMG's were not seen as being a big enough step over RCMG's to warrant their weight penalty and finally the elephant in the room, 9 out of 10 pilots couldn't hit what they were aiming at making the argument basically null and void. What might have made the difference could have been a round circle painted or otherwise fitted to the gunsight that is the size of a fighter and a second the size of a bomber at 200m and training the pilots to only shoot when the target fills the circle, at least then you won't have then opening fire at 500 plus meters which many did.
 

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