The following is BPM Inc. industry Technical document with fundamentals of Gas Pressure/port size, relative to function- in the M4/AR weapons platform.
Includes excerpts/statements /quotes from leading industry discussions in the last decade, relative to reliable SBR/suppressor usage/life expectancy of SBR type
We hope you find this information useful.
Andrew S Barnes
BPM Inc.

Bolt Speed /gas pressure in the M4/AR weapons system can be manipulated/controlled, and it depends mainly on the orifice size in the barrel, to start , before application/usage of modified recoil spring rates, or the use of heavier than normal recoil buffers, in addition to recoil spring. Gas operation is like a hydraulic system. Fluid power is all about flow (per unit time) at a given pressure. More flow, more PSI, less time are all more “work” The closer the port is to the receiver, the hotter the gas is and the higher its pressure (because the bullet hasn’t gone down the barrel to expand the volume and thereby both lower temp and pressure).


The decay rate of pressure is exponential (if you can picture one of those Quickload pressure curves). In other words, pressure drops rapidly at first, but less rapidly as the bullet goes farther down the barrel. What this means, is that the difference in port pressure and the temp is very nonlinear. The difference between a port at 8″ and 10″ is HUGE compared to the difference between ports at 16″ and 18″ even though the difference is just 2″ of the barrel in both cases.

Stated differently, the improvement of Carbine over Pistol is more significant than midlength over carbine and rifle over midlength. At a high level, there are always tradeoffs. You cannot have both a broader operating envelope (extreme heat/cold, lube/dry, weak/hot ammo, etc) AND have more durable parts. You can gain a ton of durability with a rifle gas on a 16″ barrel. But you’d have such a narrow range of functions it would be a hassle.

A likewise with the other extreme of a carbine gas on 16″ or 18″ barrels, you have a very broad range of function, but less durability. Gas port erosion is a known issue (and not a small one) on MK18 type platforms because the short barrel with carbine gas means a larger orifice size due to short plug time. The same gas length (carbine) on a 14.5 is far less severe because the longer plug time allows for smaller orifice size. Flow rate THROUGH the orifice is a major element of erosion. High flow, more erosion.

With larger ports, midlength could certainly work well with 14.5″ barrels. But you are losing some function margin. Now if you are the DoD and have communized on a handful of rounds and all of them work, great. But the home gamer running everything from 40gr varmint loads to Brown Bear to MK262 is likely to have far less luck with overall reliability.
Also, home gamers are far less likely to run something long enough to actually break parts or wear them out. In other words, a 14.5 middy compared to Carbine might offer you no advantage at
all, and actually be a detriment. If you like the recoil of the middy more, step to an AR10 spring or heavier buffer

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