You’ve probably heard or been told that most AR15s are “overgassed,” meaning that the gas port is excessively big, resulting in a variety of things we can file under the general category of no es bueno.
The TL;DR is no, milspec ARs are not overgassed. Some, many or most commercial ARs are overgassed because Joe Sixpack will complain that his H2 buffered 10″ pistol won’t cycle Tulammo in the Alaskan winter (.223 is slightly less caliente than 5.56 NATO, and el cheapo .223 is even less so).
Thus, commercial AR manufacturers learned a long time ago that it’s simply easier to enlarge the port and drop the buffer weight (saves money on tungsten) so that their guns will chew through the mildest ammo on the wintriest Alaskan night without complaining.
So where did the notion come from that milspec guns, specifically, are overgassed, overimpinged, etc.? Answer: people trying to sell doodads. Yes, it is in fact really that simple. When Colt is selling bomber rifles for seven hundred beans out the door, competitors have to do some pretty serious mental gymnastics to justify spending three to five times as much.
The truth is that milspec ARs CAN function with less gas, in MOST conditions, under MOST circumstances; BUT that doesn’t mean they’re overgassed. Milspec weapons have to function all the time, and there’s some variation in cartridges by a few thousand psi. Ammo lots are QC’d to run between a certain range, and a milspec AR has to cycle a cartridge at the bottom of that range in a dirty gun on a cold arctic night with the weapon pointed directly downwards, and still lock back.
So yes, that leaves some room, most of the time, to be able to restrict the gas flow, move the port forward and increase the buffer and or spring weight. Which makes it a little softer-shooting and a little less gassy with a suppressor. With the tradeoff being it might not cycle under certain conditions. Meaning that AR that’s been stone cold reliable on the flat range in fine weather might suddenly give you trouble if you have to defend yourself from an elevated position in the winter (think Rooftop Koreans: Detroit Edition).
If you run a suppressor full time and never take it off, there might be room for improvement. None of the military’s ARs were ever developed with full time suppressor use in mind. That said, suppressors aren’t barrels, and the impulse from suppressor back pressure isn’t the same. If you’re tuning your suppressed rifle to noticeably cut down on gas to face, you’re likely running things on the bleeding edge of what’s functional.
Finally, to play devil’s advocate, you should be looking at a flow-through style suppressor for your ARs, being that port pop is the limiting factor, especially with SBRs. That is, the sound from the port uncorking the spent case is going to be louder than the muzzle report anyways, so there’s only so much juice to be squeezed at the muzzle end. Ergo, there’s no point in having a super high efficient can that’s cutting your muzzle dBs to 120 when your port is throwing out 140 right next to your ear. Not only that, super efficient cans actually INCREASE port pop, which is obviously counterproductive, so it’s better to just get a low back pressure can that tames the muzzle blast down to baseline port pop levels, that doesn’t gas you out and that doesn’t affect the cycling of the action or require fancy doodads to switch between suppressed and unsuppressed modes.