The less-ridiculous version of this would be “metal storm” sequential cartridgeless rounds. Basically a tube that goes rifling, bullet, powder, bullet, powder, bullet, powder, bullet, powder, etc. Pull one trigger and get a very tight grouping for an absolute stream of lead. Skimp on rifling and it’s more of a death cone.
Of course the goal is probably closer to a one-gauge shotgun. You want a flying ball of death that says, fuck everything in this general direction. Ideally with the visual impact of the shotguns in Armored Core 6, because god damn, reach for the stars.