I've never had the privilege yet of being behind one.
Next to one, all the time. There's almost always at least one, and sometimes several, at the 1000 yard range we go to on the weekend.
Well, let's clarify..."next to", as in, usually staying at least a couple of benches away. If I end up near one, I always double up on my hearing protection (plugs and muffs) as they're louder to the nearby shooters than the guy driving the rifle like any rifle with a brake.
They're always courteous enough to call their shots ("fire in the hole") so you don't hit the roof if you're not expecting it.
Anyway, yeah they're loud as you'd expect. But what makes them really different (you can easily tell a .50 from the parking lot) is the concussion. They go "boom"... not "bang"...
You feel it clear through your chest, like you just shot ten pounds of tannerite in front of you.