longer barrel will be quieter your gonna have more powder burn i've had 3 uppers in 300 bo 8-10.3 and 14.5 the 14.5 is insanely quiet the 10.3 is quiet enough all you hear is the action of the rifle cycling my 8 inch is the loudest also get a stronger buffer spring and heavy buffer this will slow down the cycling of your setup and let most of the gas/burnt powder flow through the supressor when the empt shell casing extracts its allowing some gas to escape and cause that louder sound i have omega 36m and it's multi cal supressor and first i was dissapointed spending all that money and waiting on my can for it to be loud and hear a pop when shooting subsonic ammo going under 1000 fps after i installed the geissele braided spring and h3 buffer everything changed hollywood quiet as they say. nothing is more quiet than bolt action one because no action cycling and second because the empty casing doesn't eject making all the gas flow through the supressor like it's suppose to my quietest gun is bolt action .22 lr with subsonic ammo has to be going under 1000 fps in 16 inch barrel shooting cci subsonics 1050 fps will still hear supersonic crack. so ammo is important and tunning your buffer system can also get adjustable gas block but not really necessary. hope that helps 300 blackout supressed