Also alot of what your hearing is your buffer system as well as the bolt cycling and going back into lockup. What buffer system do you have? And as stated above, anything with a wipe will be significantly quieter, however the wipes themselves are a nfa controlled item(which is a sick joke in itself, disband the atf!)

I installed a jp scs into one of my Ars and it did make it noticeably quieter, thats not the entire reason they are made but a good secondary effect.

But about 10 times outta 10, yes a bolt gun will be significantly quieter then a semi gas gun while both are running suppressed. because you also cut out all of the port pop with a bolt gun, run a can with a wipe and the colest thing is after you fire and then cycle the action, you get the "hiss" of the remaining trapped gas in the system bleeding out when you cycle the gun.