High speed photos.  Note you can see the projectile in a few of them.  You can also see spokes of smoke at the end of the barrel.  These are formed as gas escapes from the grooves of the rifling a split second sooner than for the lands.  With this difference between gas flow at the lands vs the grooves, you can appreciate the importance of a crown on the muzzle.


 

 

Animated GIF of the firing sequence of an AR-15

X-Ray of an AR-15.