Heavy Barrel is a 1987 run and gun arcade game by Data East.
The story is that terrorists have seized the underground control complex of a nuclear missile site, and it is up to the player to infiltrate the base and kill the enemy leader. The name "Heavy Barrel" comes from a powerful weapon within the game that the player collects pieces of throughout the game. If all of the pieces are collected, the player can wield it for about 30 seconds.
It was ported to the Apple II and DOS in 1989 and the Nintendo Entertainment System in 1990.
The players started out with some sort of laser gun that had unlimited ammunition (which occasionally proved necessary) and a limited supply of grenades. Players might find better weapons, grenade powerups, or keys to unlock boxes which themselves might have better weapons, grenade powerups, orbs (if you had 2 players), or one of the 6 pieces of the Heavy Barrel superweapon.