Some pressure is lost through the cylinder gap (which runs around 5 thou), which is why you should never index a finger down the cylinder. A .500 S&W will cut through a finger with the gas pressure alone. The Mythbusters proved that one.

Revolver brass never stretches far enough to close that gap, if it did, it would run into the forcing cone. (plus, if brass stretched that far, you'd get head separation)

So, you do lose some pressure through the cylinder gap, but unless it's out of spec, it's not the issue you'd think. One guy at the range was shooting a Smith Model 19 with a 30-thou gap beside me one day - the pressure wave and lead spalling spitting out the sides of that gun made me wonder if it would be safer to just stand in front of it. I took him off the line until he had the barrel set back and the forcing cone recut.

Bren R.

Originally Posted By: bridgman
2. Revolvers have a small but non-zero gap between cylinder and barrel. It seems to me that given the pressure of the gases as the back of the bullet clears the gap any self-respecting gas would go out the gap rather than continuing to accelerate the bullet down the barrel, but longer revolver barrels do seem to produce higher velocities than short ones even so.

It seems unlikely (but not impossible) that the cartridge brass stretches just enough to seal the gap temporarily, but that wouldn't cover situations where a shorter cartridge is being fired, eg .38 special ammo in a .357 mag revolver.