I was reading an article comparing various conical ball rounds and it mentioned the super quiet Aguila Colibri. The review mentioned it was so quiet they wondered if the bullet was still in the barrel. I had not heard of this round before, so I looked it up. This is one Canadian supplier :
Aguila Super Colibri Armmunition has no powder, relying on only the primer to propel the bullet down the bore. The result is an extremely quiet rimfire round, perfect for close-range pest control or quiet plinking.
Muzzle velocity - 375 fps.
These rounds are powered by the rimfire primer only.
They must only be fired in handguns.
If fired in rifles, the bullet may remain lodged in the barrel.
The reviewers had shot it from a Remington 521T (rifle) so they were taking chances with that.
It's amazing the number of little weird products that exist.