Back in the sixties, when I was young, we would put a teaspoon of gasoline down the barrel of a BB gun and it would increase the hell out of the power. It would eventually rust the gun.
When I was a kid, my best friend had a Daisy model 25 air rifle. He would sometimes sneak some of his Mother’s olive oil and pour it down the barrel.
For the first few shots it would spray oil then for a few more it would shoot extremely hard. Hard enough to penetrate particle board which was what the side of their barn was covered with.