I thought “muzzle blast” was the flash and fire that issues from the muzzle once the bullet leaves the barrel, allowing flames and smoke to blast out behind it.
BTW, the video at the site is more than 16 minutes long.
I could well be wrong, but I don’t think that “AMMO” was a complete round.
I think the “muzzle blast” is the very high PRESSURE that has forced the bullet out and on it’s way. The gunpowder is a tightly confined EXPLOSION and could well do damage to anything close at hand.