How do you make instant SMOKE for a PROP GUN without noise?

[RJBowman]

I know that a cap will produce smoke in the barrel of a toy gun, but it also makes noise, and requires an apparatus that holds, hammers, and advances the cap. I had a model train that produced smoke from a drop of oil in the smoke stack, but it takes a minute for the heating element to get hot. I've also seen vape devices adapted for this purpose, but again it's slow. I need something simple; preferably no moving parts, and preferably soundless, and small enough to fit inside a derringer sized prop gun.

Suggestions?

[J. Wilhelm]

Not smoke but vapor: ultrasonic vaporizer?