57mms get 3 shots maximum for the AP shot thing if you time the gun just right. In most circumstances involving immobilized/crippled/stickied tanks, you'll get a maximum of 2 shots per use.
This is not true. You can get the sound and seeming effect of an AP round if you slam the button right as the projectile leaves the gun barrel, but do it too late and it is only the effect, and the round does not count as an actual AP round. It must be done as the gun is actually firing, but before the projectile exits the barrel for a third AP shot.
This a theoretical 3 shot maximum only, the only way you could ever get 3 shots is by sheer and total luck. In any normal instance jamming on the AP rounds button as fast as you can still will not result in the first shot being an AP shot.
