Dude there is no .416 cal Barrett .Heckler & Koch makes a version of m-16/m-4 carbine with gas piston mechanism which they sell as HK416 but it uses same 5.56*45 mm ammo and the 8mm mauser you are taking about is ww2 era 7.92*57 mm Mauser ammo which will be very difficult to find..50 cal BMG(ball machine gun)rounds have a cartridge that is 99mm in length

Seriuosly you are gonna carry it in your pocket?30/06 is the Springfield 7.62*63 mm ammo used by the US during ww2 which is also quite rare