So the Venom is a fifty, neat! 12.7mil... no definition regarding caselength or shape... so it could be a shortened beowulf... but that is still a sizable cartridge. Tt may very well be a reduced AE-style cartridge, I'm reading here that it does damage comparable to unsupressed 9mil in that game...
Why such a huge caliber at that rate of fire? The size of the bullet will negate carrying capacity per person... Drop the caliber down to 9mil and you can affectively double the amount of rounds per cubic meter per person. Lengthen the cartridge at a reduced caliber, your carrying capacity might be reduced by half to a third, but zippier-rounds mean you can effect a higher rate of fire.
Buuuuutt...... '44 era kraut metalurgy... the Venom might be about as destructive as a shoulder-fired "rifle" can be... assuming we understand its use as suppressive- anti personel weapon -only-.
Those barrels would melt at that rate of fire were they to fire 12.7x99.