Inconsistent bullet velocity during experiments can be caused by the movement of powder within a cartridge and the physical gap between the bullet and the barrel's rifling.

“The thought is that bullet is having to jump the gap whenever you fire the bullet. And sometimes we can even see it on the high speed, we can see a fireball come out before the bullet, which tells us fire is getting through that little gap right there. And we think that's giving us inconsistent velo”