Wanted to ask a very general question, because I don't quite get the rechambering of the 7,62 soviet F-22 guns to german 7,5cm rounds. I understand the chamber was rebored for a bigger cartridge, but the shell would be smaller, and that means that the propellant charge would be wasted because a portion would pass around the shell. This always bothered me.
A couple of days ago I was reading a book about automatic rifles of WWII, and it mentioned the 7,5mm cartridge used in StG 44 was directly copied to be a 7,62 russian cartridge used in AK-47. The reason for the difference in caliber was stated to be the fact germans measured the bullet size, and the russians measured the bore size. Was this true for AT guns as well? Was a Pak 40 actually a 7,62 gun? Couldn't find the answer online.