Question about Mosin ammo

[Feanaro]

So what's the difference between aircraft MG ammo and rifle loads? Bullet weight? Overall charge?

MG ammo would likely be a lot "hotter". For example, some of the surplus 7.62 Tokarev ammunition floating around was originally designed for use with the PPSh-41 SMG. It's can be very hot stuff, sometimes too hot to safely use in a pistol. Lot more powder.