ZDoom

It's not though, ProjectileKickback works in Decorate as well.
There's a chance it may also require the +PUFFONACTORS flag for bullet puffs.

I'll test that in ZScript. I can't seem to make that work in Decorate.

edit: That function is ZScript exclusive but I found a workaround. Thanks for the help!

Puffs support the ProjectileKickBack property, which will do exactly what you want.

edit: I did some research so I'm changing this a bit.

I would like a custom flag for A_FireBullets and A_RailAttack to customize the kickback of those functions. I'm aware of Weapon.Kickback, but I was hoping I could call functions that are separate from the general weapon (Say, a secondary attack or even a custom monster attack) that knocks back the enemy with a hitscan.