I have a 12 inch tromix. Runs great, but as said above, is not particularly quiet with any loads that run the action. Pistol length gas with adjustable block, have tried a couple different cans.
A 16 inch bolt gun with fast powder or longer ar15 barrel will make less noise of course... just depends on priorities.
When I got mine, the 12 inch barrel was supposedly more reliable than going short like 8 or so. Not sure if still true though.
Very happy with the 12 and would definitely go that way again. It's great for deer hunting up close in brush here.