I recently fired a full auto AR that only had an 11.5" upper and boy was it loud. The owner of the gun provides firearms for movie sets and I must admit firing a chopped down AR was fun but it was exceptionally loud which gets to my question. I currently have a 20" HBAR on my pre-ban Colt and I've been considering a a 16" bushmaster upper. Is the 16" upper that much louder than a 20"?. I've found that my 20" barrel doesn't make that much more noise than a .357 magnum. Anybody know the answer to this?

Its louder but not that much louder.  Does that answer your question? LOL!

Ive got an 11.5" upper and a 5.5" flash. If I shoot at our indoor range I sometimes get a look from the shooter next to me., Never a sneer just checkin out what I am shooting. I also shoot a Kimber Pro Carry w/ earplugs only. Cant do that with my short AR.. Too loud..