Shortround6
Major General
="kool kitty89, post: 1434647, member: 12211"].. though if necking down the existing 37 mm cartridge and using the same receiver/breech/recoil mechanism, 23 mm would probably be overkill, and something in the 25-30 mm range would have gotten ballistics more in the Oerlikon S/FF S, Hispano, or .50 BMG range while having better shell capacity)[/QUOTE]
SOme of this depends on what you call ballistics. The US .50 having a shorter time of flight to a given range than the short Oerlikons,
How you make parts depends on how many parts you are going to make per batch and how good your foundry/fabrication techniques are at a given time.
A lot of machine gun or automatic cannon parts are subject to high stresses it takes a lot of careful work to get stamped parts to work properly, it can be done but it is actually harder than machining from solid for small batches. Same with forging, you have to make enough parts to pay for the forging dies. Tough to do with small batches of experimental guns. metallurgy in 1939-45 is nowhere near what it was in the 1960s let alone today. Forging can produce stronger, tougher parts than machining from solid but without proper technique it also might not. And very often forgings have to be finish machined to bring them into tolerance.
Good chance it won't work.
From CANNON, MACHINE GUNS AND AMMUNITION
The 20 x 120 cartridge is the Madison. The diameter of the case just in front of the rim is 29mm, for Hispano it is 24.8mm and for the Oerlikon it is 24.9mm (although I would wager you might find examples that flip those numbers.) Please note the Russian 23X115 round has a body diameter of 27mm.
1. The Hispano often doesn't get quite the credit it deserves and
2. Some other rounds get a bit more credit.
The Hispano fired 130 gram projectiles at about 870-880 meters per second for a muzzle energy of of 50,3000 joules. when fired from the long barrel guns.
Which makes it the most powerful 20mm cartridge used in WW II in significant numbers. The larger Japanese 20 x 125 round fired an 162 gram AP projectile at 790m/s for 50,500 joules but the HE ammo was 127 grams at 820m/s and energy dropped to around 42,700 joules. The vaunted Swiss/German 20 x 138B round had a number of loadings but excluding the one used in the experimental early aircraft cannon none go over about 48,000 joules.
The Hispano had a decent rate of fire, anything that fired faster used a rather less powerful cartridge. Russian 20mm was around 36,000 joules and the MG 151 didn't go over 30,000 joules in most loadings that saw service, Japanese round used in the Ho-5 20mm army cannon also didn't go over 33,300 joules and was down loaded in later versions.
The question I have with the FN Browning (and it may never be answered) is what was the rate of stoppages and parts breakage per 1000 or 5000 rounds fired.
It took 3 US companies over two years to get the Big Browning from 800rpm to 1200rpm at a rate of stoppages and breakage acceptable to the US Military.
Perhaps if the target goal was only 1050 rpm they could have hit it much earlier with less redesign, I don't know. Or perhaps the countries that adopted or looked at the FN Browning were more tolerant of stoppages and Breakages, I don't know that either. The early US M2 Brownings were far from trouble free once installed in aircraft, however well they may have performed on the test bench/range. One modification needed between the 600rpm and 800rpm M2 guns was to double the force the feed mechanism pulled on the belt with. The older guns slowed down with long belt attached compared to a short one. ANd that is without G forces pinning the belts to the bottom of the feed trays. The difference in internal capacity between a 12.7mm projectile and a 13.2 in minimal, you can get as big or bigger change by varying the thickness of the steel tube that forms the body holding the incendiary material. The US went a little too far with M23 incendiary with using a minimal thickness body to hold the max payload.
The 12.7 x 81 case was too small to take heavier, longer bullets. It may have been too small to take "trick" bullets. This depends on the gun and the max overall length of the cartridge it will handle. If your overall length is fixed than bullets with pointy noses and boat tails stick down further into the case and cut powder capacity and thus velocity. A problem well known to many experimenters trying to design cartridges for the M-16 platform.
I would note that one reason the British were able to use more pointy nose on their 12.7x81 is that they never (for good or ill) designed an exploding projectile for it and didn't need to accommodate a fuse.
Anthony Williams credits the photo to "PzGr40"
You do want all your ammo to follow similar trajectories/times of flight so designing a really streamline bullet of one type and short/fat light bullet (ping pong ball) of another type doesn't work very well.
SOme of this depends on what you call ballistics. The US .50 having a shorter time of flight to a given range than the short Oerlikons,
How you make parts depends on how many parts you are going to make per batch and how good your foundry/fabrication techniques are at a given time.
A lot of machine gun or automatic cannon parts are subject to high stresses it takes a lot of careful work to get stamped parts to work properly, it can be done but it is actually harder than machining from solid for small batches. Same with forging, you have to make enough parts to pay for the forging dies. Tough to do with small batches of experimental guns. metallurgy in 1939-45 is nowhere near what it was in the 1960s let alone today. Forging can produce stronger, tougher parts than machining from solid but without proper technique it also might not. And very often forgings have to be finish machined to bring them into tolerance.
Good chance it won't work.
From CANNON, MACHINE GUNS AND AMMUNITION
The 20 x 120 cartridge is the Madison. The diameter of the case just in front of the rim is 29mm, for Hispano it is 24.8mm and for the Oerlikon it is 24.9mm (although I would wager you might find examples that flip those numbers.) Please note the Russian 23X115 round has a body diameter of 27mm.
1. The Hispano often doesn't get quite the credit it deserves and
2. Some other rounds get a bit more credit.
The Hispano fired 130 gram projectiles at about 870-880 meters per second for a muzzle energy of of 50,3000 joules. when fired from the long barrel guns.
Which makes it the most powerful 20mm cartridge used in WW II in significant numbers. The larger Japanese 20 x 125 round fired an 162 gram AP projectile at 790m/s for 50,500 joules but the HE ammo was 127 grams at 820m/s and energy dropped to around 42,700 joules. The vaunted Swiss/German 20 x 138B round had a number of loadings but excluding the one used in the experimental early aircraft cannon none go over about 48,000 joules.
The Hispano had a decent rate of fire, anything that fired faster used a rather less powerful cartridge. Russian 20mm was around 36,000 joules and the MG 151 didn't go over 30,000 joules in most loadings that saw service, Japanese round used in the Ho-5 20mm army cannon also didn't go over 33,300 joules and was down loaded in later versions.
The question I have with the FN Browning (and it may never be answered) is what was the rate of stoppages and parts breakage per 1000 or 5000 rounds fired.
It took 3 US companies over two years to get the Big Browning from 800rpm to 1200rpm at a rate of stoppages and breakage acceptable to the US Military.
Perhaps if the target goal was only 1050 rpm they could have hit it much earlier with less redesign, I don't know. Or perhaps the countries that adopted or looked at the FN Browning were more tolerant of stoppages and Breakages, I don't know that either. The early US M2 Brownings were far from trouble free once installed in aircraft, however well they may have performed on the test bench/range. One modification needed between the 600rpm and 800rpm M2 guns was to double the force the feed mechanism pulled on the belt with. The older guns slowed down with long belt attached compared to a short one. ANd that is without G forces pinning the belts to the bottom of the feed trays. The difference in internal capacity between a 12.7mm projectile and a 13.2 in minimal, you can get as big or bigger change by varying the thickness of the steel tube that forms the body holding the incendiary material. The US went a little too far with M23 incendiary with using a minimal thickness body to hold the max payload.
The 12.7 x 81 case was too small to take heavier, longer bullets. It may have been too small to take "trick" bullets. This depends on the gun and the max overall length of the cartridge it will handle. If your overall length is fixed than bullets with pointy noses and boat tails stick down further into the case and cut powder capacity and thus velocity. A problem well known to many experimenters trying to design cartridges for the M-16 platform.
I would note that one reason the British were able to use more pointy nose on their 12.7x81 is that they never (for good or ill) designed an exploding projectile for it and didn't need to accommodate a fuse.
Anthony Williams credits the photo to "PzGr40"
You do want all your ammo to follow similar trajectories/times of flight so designing a really streamline bullet of one type and short/fat light bullet (ping pong ball) of another type doesn't work very well.