Lil'gun was originally designed for the .410 shotgun, so it is first and foremost a shotgun powder. That said, I don't reload .410 shotshells, so I've never used it for that.
I have used it in my .22 K-Hornet (21" Contender Carbine) with some success. I've also used it in .45 Colt both from my 5.5" Ruger Blackhawk and my Rossi lever-action rifle. I lack a chronograph, so I couldn't tell you anything about velocities, but accuracy was satisfactory.
So, if you're shooting a cartridge that would take Lil'gun in a handgun, I can't think of a reason not to use it in that same cartridge out of a rifle.