Hello,
I've got an 11.5 5.56 SBR that's incredibly overgassed. It's a S&W M&P Sport 2 barrel that was turned down in a CNC Lathe and had the gas port drilled out to 71 thousands. I'm running the stock carbine length gas tube, carbine buffer and spring. I shoot exclusively federal xm193. Every case is bouncing off the brass deflector, which is resulting in a 1:00 ejection. There's quite a bit more recoil than when I had the 16" barrel on my gun.

What should I be looking at to fix the overgassed issue? I do plan on having a dedicated suppressor for this rifle in the future as well.

Superlative Arms gas block

.07 isn't too crazy for a 11.5.  I would put an H buffer in it.  Superlative gas block works pretty well if you just want to cut the gas.

I have a cut down 16” to 12.5” carbine gas barrel and I didn’t have to open the gas port at all to cycle Tula steel case to 4-5 o:clock ejection.  Maybe that extra inch makes the difference.  The barrel was a little over-gassed at 16”, ejecting IMI M-193 to 1 o:clock.  Not only did chopping off 3.5” improve cycling, but poor accuracy (6-8 MOA) went to acceptable accuracy (3-4 MOA or less).  It was a cheap barrel and a buddy was looking for learning projects on his second hand lathe.  I was fully expecting to have to drill out the gas port but got lucky.  

In addition to messing with reciprocating weight you could try an adjustable gas key. Costs about the same as a heavy buffer and gives a range of tuning but is more work to dial in.

Try an H2 buffer or Black River Tactical sells reduced size gas tubes.

I had a 14.5" carbine gas that I had cut down to an 11.5", left the gas port alone and put a Surefire suppressor on it. That setup is slightly over gassed, and I already have an H buffer on it.

a 16" carbine barrel cut down and opened up is going to be over gassed.

Thanks everyone for the replies. I'm on mobile so I apologize for the poor formatting and lack of quotes.

I gave some thought to an adjustable gas block (and also an adjustable gas key), but for this rifle, I would like to KISS as much as possible, since A. It's my only rifle, B. I only shoot one brand/type of ammo, C. It will stay suppressed 24/7 once my form gets approved, and last of all D. There would be fewer things to go wrong with non-adjustable components.


All that being said, I have ordered a tungsten weight set from Amazon ($27.xx) that should let me mix and match my existing buffer weights with the new ones to create anything from an H to an H3 buffer. At that point if I'm still unable to fix the rifle's gas issue, I'll look into an enhanced spring (springco springs possibly).


I'm also in the market for charging handles to help alleviate gas to the face. If you have any experience with any of the sort, I would love to hear your .02.


Thanks!

I am in the same boat.

11.3 BA Hanson Barrel. Currently I am using a strike industries flat wire buffer spring and a spike t3 buffer. I am still ejecting at the 2-230 position. Called springco and the tech told me to go straight to an orange spring. Came in this weekend and today I am going to the range. I will test it with an H, t2, and the t3 buffer. I am hoping to avoid an adjustable gas block (just don't want to add another failure point). If for some reason this does not help, I will check out the BRT gas tube. I even bubba'd my BCM charging handle to try to vent gas going straight up my nose.

Hoping for some luck today when testing it all out.

I wanted to follow up, sorry for being late.

I put in the orange spring, and the 5 coil extractor spring. I am now ejecting at 2:30 to 3:00. The hole in the side of my charging handle helps a ton as well. I am not thrilled that I am still over gassed and can not slow down the bolt speed, but it is significantly better than it has been since I have built it.

I know that my next step is an adjustable gas block or the BRT gas tube, but I am not in a rush at the moment. I have a lot of cam wear. Pistol has been shot for maybe a little over 1000 rounds and the cam is pretty polished. Another step I may take is contacting BA and prying some info about the gas port size. I would guess that many of these companies over gas these shorter barrels for reliability, which in turn makes suppressing the gun not as fun. I will update as I find new information. I don't think anyone is at fault, this is only a learning experience for me.

When you add the suppressor it will be worse.
I have a 11.5 and 12.5 suppressed, I have adjustable gas.

12.5 is mid gas with .055 hole.
11.5 is Adam Arms piston carbine gas, they say suppressed setting is 1/2 gas what ever that means. In this setting it won't cycle without a can.

For gas in the face problem I use the Geissele airborne charging handle.
If you look at it, it has a higher ridge than most charging handles so it helps a lot.

You could go to an A5 buffer system. A lot of people on TOS say it helps smooth out suppressed applications. Have you tried RTV around the charging handle?

A 0.071 port isn’t crazy for that application. It’s smaller than a factory Colt Commando or BCM at 11.5”

You are on the right track.  Mr. Stoner's platform, as we all know, is gas operated (and I vividly remember one kid when I was in basic training at Ft. McClellan in 1982 wanted to know what kind of gas and where you put the gas in the rifle...lol...yes, a cliche for sure and he actually asked that question)..anyway, it's a system, and like any system, if you change a parameter, you have to make adjustments in the system to account for it.  Just like your suped up hotrod, you don't put higher flowing heads and a 4-barrel carb on the engine without increasing exhaust flow.  A change on one end requires a change on the other for it all to function smoothly.

I've said it before, but in a perfect world, the AR-15/M-16 would be perfectly tuned when it runs reliably with the lightest buffer and spring possible.  Why? because in that profile, that equates to less gas, and less recoil.  Less recoil is always a good thing, right?  In order to achieve that, it means you have to control the amount of gas flowing in the system.  The pressure will always be a function of powder, specific cartridge, port size, and dwell time.  Technically, we have control of every one of those, but some are easier to change than others.

Sure, you can pop in a heavier buffer and/or a stiffer spring to mitigate over gassing, but in reality that's a bandaid on the main issue...too much gas...and, it means heavier recoil (check out what the 9mm blowback upper guys deal with).  

Of course, as with all things, it's never as simple as changing out a component or two as everything has a trade off.  The short answer is an AGB...but those have their own issues with pistol length gas.  They wear out quickly, or develop adjustment problems due to the higher temps, pressures and amount of crud that flows through them.  It may well be a "thing" you deal with and replace as needed, or you can try other solutions.  The BRT tubes are one solution, or even a PRI fatboy tube, same basic principle.  They increase the tube size as gas flowing from a high velocity, small tube into a larger tube slows down and decreases pressure (and it also cools it as well).  The only issue with BRT is that you have to be sure to get the right sized one as you can't adjust it.  An adjustable BCG is also an option, or the Innovative Arms W.A.R. receiver with it's little knob, but it has only two settings, full gas and restricted...and it also does not work (according to Innovative Arms) with pistol length gas.  Thgen there are the piston conversion kits, like the Adam's Arms, but that brings it's own set of issues as well (carrier tilt)...again, everything has a trade off.

I think most manufacturers over gas on purpose to ensure functioning, they have no way to know what ammo anyone will use, or any other devices, like a suppressor.  That's one thing for sure, over gassed is more likely to function than under gassed.  Either way, the AR has gone through many changes and developments due to its popularity.  Rifle length, carbine, pistol length, different gas systems, different cartridges, suppressors, different manufacturers all tweaking and changing things to make a better product than the competition...and all of them still operating on the same basic gas system.  I think 300 Blackout shooters have the worst conditions over all...because the cartridge has such a broad range of bullet weights and it's popularity in both super and subsonic uses.  That's a huge spectrum to try to manipulate for functionality's sake.  The absolute best combination of pieces parts uiltimately resides in a purpose built firearm.  A pistol length, suppressed 300, for example, can be built to run smoothly, with the least possible recoil with the least gas needed, but then trying to shoot super sonics will throw it out of whack.  Even with a combination of adjustable gizmos you might be able to tune it to at least run decently, but it will never be perfect or ideal.  If you don't care, sure, you can keep it over gassed and shoot away, but then you're beating the heck out of the gun for no reason.  For a relatively simple design, it's not so simple given the vast amount of variances introduced over the years.

Ok, Update:

I swapped the H2 buffer for an H3 and got a Bootleg Inc adjustable BCG. That combination fixed my overgassed problems immediately on the "unsuppressed" setting. Overall not cheaper than just getting a new barrel or an adjustable gas block, but it provides future adjustments once my stamp for my suppressor gets approved.

Alright, I’m going to hijack/add to the discussion.

I have an 11.5” upper with a superlative arms AGB that is over gassed. I always run it suppressed but I have the screw on the AGB backed out entirely (bleed off mode), 100%, and it is still over gassed. With an H3 buffer and AR15 BCG, it throws brass at 3:00.

What is crazy to me is that I have an almost identical setup (10.3” barrel and Superlative Arms Piston kit) and that system is tunable, runs flawlessly and I only require a standard buffer. This one is also suppressed. FWIW, this is the smoothest, lowest recoil AR I’ve had the pleasure of using.

I shoot XM193 out of both.


Kind of chaps my @$$ to use the Superlative AGB in restrictive mode rather than bleed off since the bleed off functionality is what makes that gas block unique. But maybe I have to.  My goal with that build was to create as light weight of a system as possible, so I started off with a skeleton BCG and carbine buffer and have worked my way up to the BCG/buffer I’m currently using.

Anyways, insight is welcome. Were it not for the nearly identical piston kit system tuning mid range and running flawlessly, I wouldn’t think much of it.

Bleed off may be unique but it is more difficult to balance a pressurized system with one input (Gas port) and two outputs (bleed orifice and gas tube to gas key).  I suspect the bleed orifice is sized for the valve flow characteristics and “typical” pressure vs time inputs. Your ammo, barrel, and suppressor just happen to fall outside that functional range for the bleed implementation.  Troubleshoot the gun in restrictive mode and if you get that working it’s all good, that’s what that mode is there for.  You should be able to restrict to the point a lightweight carrier system doesn’t get beat to shit. Sometimes over gassed systems can look like under gassed so start choked down to straight pull bolt action and open up to approach the operating point.  

Similarly, adjustable bolt carriers bleed off at the bolt carrier and while they can work well for right handed shooters to have switchable bolt velocity, they can jet gas into left handed shooter’s face.  Some people find the fixed settings on adjustable bolt carriers lack fine tuning resolution.  

Bleed off makes sense for piston systems because the gas has to go somewhere quickly to allow for piston reset.  For the Stoner gas system throttling at either end of the gas tube (adj. restrictive gas key or gas block) works well until wear changes throat or carbon/corrosion locks up adjustment. I’m not sure if there are similar wear issues with SA bleed off mode as the ones I’ve seen amongst acquaintances don’t get shot much.  I do know my friends had trouble dialing them in, but more for lack of operational due diligence (RTFM aversion).

Now that you have the adj bootleg, drop the h2 back in it and go out, I bet you can turn it to all the way suppressed or 1 click back and be even better

H2 buffer and springco blue spring.......thank me later

This is not the answer. It may be an answer for your barrel depending on the gas port size. I am using a H3 and springco orange and I still eject at 2 o'clock because of an over sized gas port. Controlling the gas amount is the answer. I shot my pistol with a can in a class yesterday. We did some flat stock exercises.... Guess who got an over bearing amount of gas to the face through the ejection port? This guy. Even took some shots with my eyes closed at 5 yards because it was burning the fuck out of my eyes since you shoot the gun sideways with your nose in front of the CH if you can.