Germany got extreme ballistics on an artillery piece, by staging powder charges along the length of the barrel. As the projectile passed, the charge exploded from the previous explosion. It was called the V-3 gun.

https://en.wikipedia.org/wiki/V-3_cannon

A single secondary charge in a rifle should provide the same boost in velocity they saw. Think what 3800 FPS with a 220 grain bullet could do....

:)>