Most linears do a good job. I use BRT Covert. However, be aware that in an enclosure there is no getting away from the peak noise levels. If inside a building or under a shooting canopy at the range the pressure exhausted behind the bullet is simply directed, not reduced. Given the way the ceiling, walls, and floor redirect the blast wave it may not seem to help at all.
I have fired my AR pistol with the Covert and anecdotally it seems to be less noisy in open air than "normal," whatever that means. Under a shooting canopy all bets are off tho. At that range the canopy slopes downward at the front where a blast wave is directed back at the shooter. I usually shoot at one end where I can get out from under it when others shoot, and when "on the line" tend to stand forward of the front edge or down at the 7 or 10 yard line.
It's as if they deliberately tilted the slope to make it louder. There's no helping the side wall construction, six foot vertical concrete walls. YMMV.
