Since we are on the subject of gas flow. This is a Krieger .223 AR barrel that I chambered and crowned a few years ago (4 groove with 12 deg. crown). I indicate off the grooves for this process. After a cleaning, this nice pattern emerged after 5 shots. This is fouling mixed with Shooters Choice residue.

I would love to see a close-up slow motion animation of the gas flow here. I just can't imagine what causes the little spits of flow at the edges and center of each narrow land, but there must be some sort of swirling going on. Are there any folks here who work in high velocity gas flow? Any opinions? Food for thought.