I always wonder why comic writers/artists feel the need to give superpowered characters guns.
Venom has all the abilitys as Spiderman and Spiderman has never needed to use a gun, so why give the new Venom a gun.
They've actually covered that fairly well in an interview here. Basically, the army is kind of using Venom as a weapon/exoskeleton. (yeah, that will end well...)
It's probably happened because Marvel has been spinning their wheels creatively and just relying on Deadpool (who I will try not to bad-mouth) to sell comics