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- #121
A better thought-out '2nd gen' of German 2-engined A/C?
- Thread starter tomo pauk
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Shortround6
Major General
The facts on the shell weights and velocities of the historic cartridges are real.
What could be done with modifying them gets iffy.
From Anthony Williams' book Autocannon.
20 X 80 standard (134g) projectile, 600m/s using 13.5g of propellent and about 280 MPa chamber pressure.
20 X.80 Mine (92g) projectile, 700m/s using 15.5g of propellent and about 280 MPa chamber pressure
20 X 80 standard (115g) projectile, 585 m/s using ?? g of propellent and about 280 MPa (?)chamber pressure Fuse was changed from brass to aluminum which accounts for weight difference.
20 X 82 standard (115g) projectile, 710m/s using 19g of propellent and about 285-310 MPa chamber pressure.
20 X 82 mine (92g) projectile, 800m/s using 19g (?) of propellent and about 285-310 MPa chamber pressure.
20 X 70/72 standard 128 g projectiles, 600m/s 13.5g of propellent
20 X 99/101 standard 128 g projectiles, 750m/s 21.4g of propellent
20 X 110 Oerlikon standard 128 g projectiles, 830m/s 28.5 g of propellent 310 MPa chamber pressure
20 X 110 Hispano standard 128-130 g projectiles, 850-880m/s 32.5 g of propellent 335 MPa chamber pressure
Not all propellants are the same burning rate although the energy per gram should be similar.
No information on the 15 X 96 powder charge.
The 15 X 104 BESA gun cartridge could fire a 74-75g AP projectile at 884m/s using 24 g of propellent (NC) at 334 MPa or 303 MPa depending on if the 22TPI is UK or short ton.
US .50 used about 15.5g of propellent for anybody that is interested.
Propellent charges varied a bit depending on exact propellent used and the desired result. Changing weight of recoiling parts (barrel/bolt/etc.) and shell weight and call for a different propellent with a different burning rate. However the gun/ammo designers still have to keep to the same chamber pressures or run into accidents. The API guns used long chambers to keep the cartridge enclosed during the initial travel backwards so the cartridge walls were supported. If the case head clears the chamber with too much pressure you can/will get case ruptures which at best, ties up the gun until serviced by an armorer. At worst you need some new parts or a new gun and possibly new access hatches.
If you want to push the working pressure you can cheat a little bit by using thicker case walls and making sure you quality control is really really good.
US had three grades of 30-06 ammo in WW II. Standard, Machinegun and Aircraft Machinegun. Only difference was the quality of the brass in the cartridge cases to reduce the number of extraction problems or ruptured cases. A jammed rifle was only a problem for the individual rifle man. On the other end a bad round in aircraft machine gun cannot be fixed until the plane lands.
Necking rounds for API guns down to smaller diameter is not a very good idea.
1. The primer is going to go off before the case shoulder hits the front of the chamber, which is going to mean the shoulder tries to go forward. It also means that the neck gets expanded outwards then tries to get squashed back to the right size as the neck reaches the end of the chamber.
2. Extraction is also going to be a problem, The locked breech guns keep the cartridge in the chamber for at least part of a second to let the pressure drop. The API blowback is starting to open quicker and the shoulder/neck is going to be expanded during the initial extraction/ This can lead to splits/tears in the case and possible case separations.
I don't know of any API firearm that uses or used a bottle neck case, from aircraft gun to submachine gun.
Most of the Oerlikon guns used rebated rims so the rear of the case would have no extractor cutouts (groves) the case could expand into during the initial extraction.
- Thread starter
- #123
Okay.
Thank you for listing out the details.
The table from here gives 13.5g of propellant also for the 115g shell. It also says that the FF will have about 23% greater muzzle energy (column before the MV, expressed in 'mkg' units) than ther FFM with both firing the HE, or some 8% more than the FFM firing the Mine shell. FF will also have a greater momentum - mass_x_velocity indeed - about 20% greater than the FFM (meaning that new ammo will not cycle the FF; conversely the FFM, with it's less strong return spring(s) will be getting beaten and probably wrecked after just a few old rounds fired).
Leaving aside the M-shell, one wonders why the FFM was not getting greater MV due to firing a lighter HE shell by, what seems, the same propellant load, and from the same barrel length.
The big Oerlikon (S, FFS) used the bottle neck case.
picture
And we can always take the middle path (caliber).
The ZB-60 is available from 38 (for the Luftwaffe), admittedly a relatively small cadence and not very featherweight, but these are land variants. Maybe air models would be lighter. After all, the first MG151s were in the same 15 mm caliber.
A little off topic - Bf 109F with the engine slightly moved forward (a la Ki-61-II) and 3x15 mm.
The ZB-60 is available from 38 (for the Luftwaffe), admittedly a relatively small cadence and not very featherweight, but these are land variants. Maybe air models would be lighter. After all, the first MG151s were in the same 15 mm caliber.
A little off topic - Bf 109F with the engine slightly moved forward (a la Ki-61-II) and 3x15 mm.
Shortround6
Major General
For some reason the propellent may not have been the same. It might have weighed the same (or very close) but had a different burn rate. Which affects the whole pressure curve and point of peak pressure in barrel and the area under the curve and the amount of pressure in barrel at the moment of exit. Or they may have used the same actual propellent and accepted that it was not ideal.
Smokeless powder is strange stuff. The more pressure it is under the faster it burns. So changing the pressure in the first few cm of projectile travel can have large consequences.
Now due to inertia a heavier projectile can have higher peak pressure than a lighter projectile using the same powder charge. However that doesn't always mean higher velocity at the muzzle as the peak pressure may have occurred earlier in the projectile travel.
Using different powders can also affect the amount of muzzle flash.
Just throwing out possibilities.
Lets also remember that the late 30s saw some changes to powder/propellant technology. The US gained almost 400fps on the .50 cal gun at the same pressures using a new powder that was adopted for service in 1940. The .30-06 was able to get the same veleocity using the same bullets in late 30s compared to WW I using powder that peaked at 42,000psi instead of 50,000psi. The powders used the same 'technology' but were not the same. Filling the .30-06 full of .50 cal powder would have resulted in less velocity. Using the .30-06 powder in the .50 would have blown the gun up before you got the desired velocity. Where the Germans fell on this I don't know. It wasn't exactly secret. The powder (at least the stuff for the .30-06 ) was available to civilians before 1939 and could have been purchased at any gun store.
I would also note that the Germans started work on the MG 151 in 1935, and like a few other things (other guns and engines) it took a bit longer than expected to get things to production status. How much effort was going into R & D on the MG/FF but that is part of your 'what if'
there was some, but could there have been more ? and at what cost to something else. I would also note that the MG 151 is a bit under rated in some listings as it was actually a bit lighter than usually credited for. The usual 42kg includes the belt feed, cocking system and firing system (remote trigger). Bare gun was 36-37KG. Weight of the 60 round drum on the MG/FF was 8.2 kg empty. weight of the 90 round drum was 12.0kg empty. Installed weight was closer than it seems although the 'boxes' for the ammo belts for the MG 151 do have to be accounted for.
Mr Williams book does describe an experimental MG/FF Ausf L using ammunition loaded to 20% higher chamber pressure and a 50% longer barrel to get 820 m/s (ammo not stated but guessing mine shell?) weight went to 32 kg.
Thank you, I an only blame old age and brain fa*t.
Shortround6
Major General
Ballisticly not a lot different than the 15 X 96 round used in the MG 151/15. Since the MG 151/15 was both lighter and faster firing, this explains the lack of interest before we even get into the NIH.
Yes, I agree, but the difference of two years (1938-1940 for MG 151/15 to start production) makes the difference "better a sparrow in the hand than a pigeon on a branch" as the saying goes.
Even BESA started license production in 1939. Where did the development of the MG 151, which started back in 1934, get stuck?
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- #128
The Ausf L was too late (late 1941?), indeed it was to operate at 3600 kg/cm^2, vs. the 'normal' FFM operating at 2400-2600. The 'L' was also to fire at 600 rd/min, vs. 520 rd/min for the FF and FFM.
There is no doubt that MG 151/20 was a superior weapon.
The MG FF and FFM have had the advantage of being there almost 3 years earlier, and in case where space was too tight for the 151 to be installed. The MG 151 might require sturdier/heavier mounts.
FWIW, here is what Fw was counting on wrt. different weapon sets for the Fw 190 (including the MK 108s to fire synchronised - ??). Pair of belt-fed MG FFMs was supposed to be lighter than the outer-wing pair of MG 151/20s by almost 30%, despite having extra 10 rds of ammo; the FFMs were also to fire at 20% lower RoF, and have extra 1.4 sec greater duration of fire (worth perhaps an extra burst, if that much at all).
(we can also see the 10% reduction in RoF for the synchronised 151)
Single belt-fed FFM installed (ie. including the mount and other ancillaries) was to weight some 45 kg, while a single MG 151/20, unsynchronised, was to weight 67 kg. The belt-fed FFM 'avoids' the weight of the drum, that went to 8.2 kg for 60 rd, or 12 kg for 90 rd.
Seems like the 'no free lunch' rule is/was still alive and well
Shortround6
Major General
The Big Oerlikons tended to have a bit of a problem.
The shoulder and neck tended to disappear to a greater or lesser extent when fired.
If the rounds were suitably greased and everything went went well, then problems were minimal (no worse than anything else?) However things going well include quality control on the cartridge cases as to the alloy and heat treatment. How important this was is somewhat reveled by the post war use of the Oerlikon guns in the US Navy when the US had both steel and brass cartridge cases. The US was also putting in barrels with fluted chambers, much like some of the 7.62 X 51 battle rifles of the 1950s and some LMGs. For the US that worked well enough in the Oerlikon guns that they didn't have to grease the 20mm ammo anymore if they had the fluted barrel and the Brass ammo. Steel cases still needed to be greased.
Trying to neck the 20 X 80 case down to 15mm (and lengthening it to somewhere in the low 90s?) might have been a bit too much with cases shoulder and neck blowing out.
Granted the 20 X 110 used a bit higher pressure but the cases were never going to stand up to normal chamber pressures.
Most pictures are of unfired cases or cases that were obtained from broken down from unfired ammo.
There is going to be variations in how far the case was blown out. Individual rounds can vary in pressure, different temperatures and even different amounts of grease on each round.
Hot gun vs cold gun.
Trying to find pictures of the guns in action is hard and we need to blow the picture up quite a bit but the spent cases on the deck do look different than the unfired cases.
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- #130
The information available seem to be very limited, however seems like Japanese managed to get a 14.5mm shell of 44.7g (vs. 52 g on the 151/15) to go to the pretty respectable 970 m/s, with a gun that is comparable in weight with the MG FF, ie. 2/3rds weight of the MG 151. RoF was also not bad, 630 rd/min; ammo feed was a 100 rd drum. Rebated rim cartridge, 14.5x100RB.
table
Also from here:
Robert Mikesh has created another of those remarkable books from Schiffer, and unlike most Schiffer books it is farily free from typographical and grammatical errors.
Japanese Aircraft Equipment, 1940-1945
In the chapter on machine guns, he notes that the Japanese called anything over 11 mm a machine cannon. So the standard heavy machine gun of 12.7 or 13 mm were, by their reckoning, cannon.
Indeed, the Navy Type 3 was 13.2mm. There was even a Navy experimental gun (14-shi) with a 20 mm Oerlikon necked down to 14 mm. I suppose that would rate as a cannon by both the Japanese and the western definitions.
SpicyJuan11
Senior Airman
Dornier proposed a similar a/c in 1937 but was rejected by the RLM
