I did some messing around with the game files and made an Iron Slug that shoots rockets.
Model: rsrc/character/npc/monster/gremlin/artillery/model_cannon.dat (Mortafire gun)
Uncharged shot: rsrc/character/npc/monster/rocketpuppy/parts/compound_rocket.dat (Rocket puppy projectile)
Charged shot: rsrc/item/live/bombshell/model_bullet.dat ("thrown bombshell" items seen in Compound 42, etc)
Uncharged shot impact effect: rsrc/particle/fx_explosion.dat yellow tinted explosion)
Charged shot impact effect: rsrc/particle/super_explosive_block.dat orange-tinted explosion)
I replaced the effect where a charged shot hits a wall with the effect for a heat orb evaporating, too. This is because, curiously enough, when a charged Slug shot hits a wall before exploding, it vanishes and doesn't make any noise.
Further, for the sounds, I replaced the firing noise with the gun puppy bark, and the "hit" sounds with the bomb-block noise.

This is a relatively simple weapon to make. I'd like it to be craftable somehow, or like an 8% drop in a promo box instead of the disgusting <1% chance for that stupid orbitgun if you're gonna take the easy way out. Equinox leaves a bad taste in my mouth.
Anyway, a further note is that this weapon is incredibly distracting, but so is the Big Angry Bomb and nobody uses that either. A follow-up request of mine would be to please fix the Magnus line's knockback on the charge shot.