Gosh I wish I wasn't so thick...
Ok, so I now understand some of the differences. Here is my bottom line question which I hope will get to crux of the situation.
This MAS 49/56 shoots .308 Win ammo. .308 Win ammo is 7.62 x 51 or "NATO" ammo. I've got 7.62 x 51 ammo in my possession so I'm assuming that I'm good to go. I DON'T't have 8mm ammo.
I didn't buy 8mm mauser at all so let's discount that idea. Yes, It would be very bad if I tried to stuff a large round down a narrow barrel. I now understand there is a huge difference in 8mm, .308, 7.62 based not only on the size of the round but the charge and case. But, if I've got 7.62 x 51 ammo which is a milsurp NATO round, and my rifle shoots .308 Win, which is a 7.62 x 51, then I should be ok. Right? Am I still missing something? I know there there are differences even between manufacturers. I know that Federal, for instance, produces 60,000psi while the milsurp has around 50,000 psi. The original 7.5mm round that this rifle originally shot was slightly lower. So, I'm to stay away from high pressure rounds and stick with the .308 NATO rounds. Am I finally getting it?