Folks it isn't "gas cutting" and it isn't a "gas check", per se.
Lead bullets, and even jacketed bullets for that matter, have a contact point with the bore were friction is at its greatest; gas has nothing to do with it and its a function of pressure and friction.
Read again; gas isn't doing anything, its pressure and friction that causes leading......
Quit with the "gas cutting"; as you can see from the above bullet, the area with the most stress with pressure and friction is what's getting deformed and melting; other parts exposed to "hot gas" aren't doing anything.
They were called a "gas Check" long before you knew everything, so please...