About 15 years ago I picked up a brick of Aguila Colibri ammo. 20-grain bullet, no powder charge (just the priming compound). Have used those in several handguns (revolvers and semi-autos) with good results on nuisance critters at close range. Very quiet (less noise than my old .177 air rifle).

Will not cycle semi-autos. Not recommended for rifles (danger of bullet lodging in the bore).

The Colibri is more like the BB caps. Aguila also made the Super Colibri, more like the CB caps.