On average, how much velocity is lost when...?

i did a bit of research on this, i questioned the velocity loss from a cartridge shot from a revolver due to the cylinder gap. from all my research (done on the internet) it doesn't seem to matter or there is very little loss. if you watch a slow motion film of a semi-auto you will see the projectile leaves the barrel before any gas escapes or any movement of the slide.. i agree with the one percent or less loss of velocity.