I'm sorry but anybody saying m4 or anything in an intermediate cartridge hasn't been paying attention to the show. You got your big ole polar bears and the 'guardian', which sounds like it's really big, really heavy, and like it has a bunch of metal on it. You want something that you can use to punch through from a good distance away.
.223 isn't enough. 12guage doesn't have the range you want. 50bmg may be nice, but the gun is too heavy to lug through the jungle and your magazine capacity is limited. So it really calls for something reliable in either .308 or 30-06. Either a fal (para for me), a saiga with a pistol grip & folder, garand, m1a, etc. 20 round detachable magazines with a total of 200 rounds = 10 mags. 1 in the gun, carrying 9 in a vest / pouch. Simple, powerful, reliable, carry all your ammo with you and qucik mag changes / follow ups.
But if you had the .50bmg, you're probably going to be able to get the hatch open