I see is the air in the barrel under immense pressure being shoved out the muzzle by the bullet behind it.
It's smoke.
All cartridges/barrels leak gas as the brass expands to release the bullet, but before the bullet seals the bore. With oversized bores/undersized bullets the leaking may not ever stop. Tighter chambers can lessen thisphenomena.
Back in the days of iron sights & service rifles, there were times when I could actually see the gas come out the muzzle first and the bullet pierce the cloud, creating a smoke ring. I guess those are relatively loose chambers, because the gas volume was much greater than in those videos.
