Yep you were hearing the muzzle blast being redirected by the brake on the ARs.
The minis and the boltguns most likely werent wearing brakes and you were actually hearing the sonic crack of the bullet before you were hearing the echo of the muzzleblast, which is what gave it the drawn out 'overlapping cracks' effect instead of the KABOOM (ouch my brain is wiggling) effect.
BTW, the brain wiggling comment is originated with my 8 yr old daughter being closeby when i used to shoot my .30-06 w/ muzzle brake. Shed say it was making her brain wiggle thru her earmuffs.
Arent kids great. They have the best way of explaining things.