[Tony Williams]

Originally Posted by Harri Pihl

I'm not arguing wether USAF swould have done better with something else than the M2. I merely replied to your point that the Germans and Russians went towards lower velocity, higher caliber weapons, my point being that the USAF stayed on higher velocity weapon, the M2, and even did consider another high velocity weapon, the 15mm MG 151, which was replaced by lower velocity weapon in Germany. In other words I'm saying that the air forces selected weapons with different standards and the USAF experience is just as relevant as German and Russian experience because the USAF results are undeniable (actually same can't be said about German results).

Infact I agree with you that the USAF might have done better by adopting a cannon like the Hispano or the MG 151. However, it's not the point discused here.

To be precise, the M2 was a moderately high-velocity gun (the muzzle velocity was little more than the 20mm Hispano's) and they rejected an extremely high-velocity one (the T17 - the MG 151 copy - was chambered for the US 15.2x115 round, far more powerful than the MG 151's 15x96).

I suspect that there were two reasons why the US stayed with the .50: it was the only reliable aircraft gun they had, and it proved adequate as long as they fitted at least six of them. That is not exactly a ringing endorsement.

The following extract from a 1944 US evaluation may be of interest:

"As it is now, we have the 50-cal. gun which has reached its peak. The only improvements will be minor. The only good increase is to increase the number of guns. So it seems to be just about the right time to look for a better weapon. There are two possibilities here - the one we have and the one we might get shortly. The one we have is a 20-mm gun. I think very highly of it. In fact, it is one we have here, and it is one in hand. It won't do what the 60 will do, but we haven't got the 60, and we won't have it for a year. So, we are gradually working into all of our aircraft the 20-mm gun. To give you some idea of the 50 versus the 20 and dispel a lot of ideas that have bothered us, I would like to give you a comparison. When somebody goes from four 50's to two 20's, to the layman that means a decrease in fire power. Actually, quite the reverse is true. In the horsepower of the gun, one 20 is equal to three .50-calibers. In the actual rate of fire delivered at the target, one 20 equals three 50's; in kinetic energy at 500 yards, one 20 equals two and one half 50's.[N.B. This takes no account of the effect of the HE content of the 20mm shells]

That adds up to four 20's equaling twelve 50 calibers, judging by those standards. Of course you have other advantages of the 20. You have the much greater penetration of armor. The 20 will go through 3/4 inch of armor at 500 yards, while the .50 cal, will go through only .43. In addition to that you have one more great advantage - that is you can have longer and more frequent bursts without damage to the gun with the 20 than you can have from the .50 cal. That is important for the strafing airplane, because they are burning up their barrels and ruining their guns on one flight. Sometimes it is long before that one flight is over. They will come down with screaming barrels and get trigger happy, and then all the barrels are gone in one flight. It should not happen in a 20mm. Of course, you have disadvantages. You have a heavier installation, one-half as much ammunition for the same weight. Our standard ammunition in the Navy is 400 rounds in one gun. The Fleet has set up 30 seconds of fire as a minimum requirement for the .50 cal gun. We can't do that with the 20, so we give them 200 rounds. The 20 is lethal enough to get far more results out of that 200 rounds than the .50 ever will out of 400 rounds."

From: "USN Report of Joint Fighter Conference NAS Patuxent River". (October 1944)