I do admit that I have fired some of the little primer powered CB cap ammo inside my house, down the hallway, from a bolt action 22 for fun or just to see if the gun will light a primer. They were the Mexican made Colibris or such what. No penetration in a board backstop, and no noise to speak of, but the do make a little smoke so it was pretty much limited testing. Also tested 22 revolvers indoors with that, but just enough for function testing. I have also fired some of those plastic cased plastic bullets from a 38, the kind that just take a primer to propel the "bullet". They need a good backstop indoors. Lot noisier than the primer powered 22's. But I never was overly concerned about the primer smoke. Good thing it was pretty much a limited experiment. You could get away with firing rimfire in your basement in a neigborhood, I guess. I might try it if I could ventilate the place, but all it would take would be one round going awry, or nosy Mrs. Kravitz peeking in your basement window and...