Don't even try to chamber a 7.62 Nato round in a 7.62 x 39 barrel or vise versa. Before you even start a build you'll need a lot more knowledge of firearms than you have. Hang around and do a lot of research. Building a firearm is a highly technical undertaking. Just because things fit does not mean they will function properly. Each time you fire a gun, there is an expolsion powerful enough to blow your face off and turn that gun into lethal bits of shrapnel. there is upwards of 60,000 lbs of pressure in some guns when fired. 2000 times the pressure you put in your car tires. Do you want to lay your face on a tire and have it blow-out???
"War is an ugly thing, but not the ugliest of things. The decayed and degraded state of moral and patriotic feeling, which thinks nothing is worth war, is worse. A man who has nothing which he cares more about than his own personal safety is a miserable creature, and has no chance of being free unless he is made free and kept so by the exertions of better men than himself" John Stuart Mill