I was told the Army specs on the 20" M16 is 100psi on the gas key with the full length gas tube, and 125psi for the 14.5" M4 with the carbine length tube....because of the short length of barrel in front of the gas port, the amount of dwell is reduced so DPMS opened up the gas port... so don't know if this is the answer unless you are shooting open sights... I don't have personal experience... but my game gun is a 16" mid and my back up is a 16" carbine length... there is a perceptible difference, but there are options if you want to reduce recoil impulse to the minimum amount... a 4oz buffer is a good start and a good multi port brake like a Miculek...for a full length rifle stock, if you remove two of the steel weights in a rifle buffer, and replace with a piece of aluminum spacer, you get a pretty reduction of scope movement... or an H2 buffer if you shoot a collapsible stock