I have assembled a rifle, primarily constructed of Aero parts.  It features an 18" .223 Wylde chamber and a rifle length gas system.  I'm pretty sure it's under-gassed as the rifle performs well for about 40 cycles, then starts short-stroking (cycling far enough to reset the FCG, but not far enough to grab the next cartridge from the magazine).  I've changed complete lowers, buffer systems and tried just about everything to resolve the problem, but it has not been resolved.

I'm going to assemble a different complete upper, this time with a 16" barrel.  I've read too much about gas systems and dwell time and such to confuse myself.  One source suggested that the *best* (in their terms most-closely compared to gas-pressure in the M4) is a 16" barrel with a mid-length gas system.  I'm looking at a pair of barrels; both 16" both chambered 5.56 NATO, one carbine and the other mid-length.  

I'm hoping to build a reliable blaster.  A machine that will consume any .223 or 5.56 NATO it is fed; but without over-gassing and prematurely damaging components.

I am asking for the collective wisdom of the experts.  What say you?

Which would be the better choice?  Carbine or Mid-length?

I have a 12..5" with mid gas and port sized for suppressed, works well with standard carbine buffer and spring.

ETA: I like the gas block being farther out, it gets warm suppressed plus easier to have longer hand guard.

My first are 16" with carbine system. Reliable, but that system does take a beating because that combination is way over-gassed. My main is a 16" with mid system. Still reliable and still over-gassed. I installed an adjustable gas block and it is only opened 5 out 16 detents.

How are we determining that barrels are undergassed or overgassed ?    You haven't been able to tune the recoil control to function reliably by changing the recoil spring strength and/or the buffer weight ?

I would wonder if either the bolt tail is undersize or that the BCG gas ring run is oversized and the bolt carrier group is gas inefficient, or that the BCG or bolt itself is out of tolerance and the heat of 40 rounds makes it bind during operation.   I wonder if the gas tube to gas key fit is poor, or that the gas key is improperly installed and is leaking.  

I would install the spare BCG and vary the buffer weight to see if the problem gets better or worse.  

I have several carbine length uppers and a mid length gas upper.   I prefer the midlength gas system, but have no issues with any of them.  


An 18" mid length gas barrel arrived recently, and I look forward to assembling and testing.

Maybe we should begin with what ammo you are shooting.  If bulk .223, then a rifle tuned for 5.56 may short stroke.  If tuned for .223 it may seem overgassed for 5.56.

Your choice of a rifle length gas system with an 18" barrel seems reasonable.  As others have suggested, check to make sure your gas block port is centered on the barrel journal's gas port to maximize flow and that the gas tube is positioned correctly so that you don't have a leak or obstruction. I'd do these things before changing barrels.  If all is ok and it still wont cycle low power bulk .223, then the gas port on the barrel may need to be opened a little bit.

Just to eliminate variables, try different magazines if the bolt cycles and ejects but is not picking up the next round..

FWIW,  I have 16" carbine gas port barrels, two of them, that cycle everything reliably, but some would call them over gasded.  One is NATO, the other is a Wylde stainless match barrel.  Chamber configuration has nothing to do with your issue.

Nothing wrong with a rifle journal on an 18".  It should shoot softer.  Just check for gas flow as outlined above.

Up to this point, I've tried the following things with the 18" rifle-length gas upper-receiver assembly:
- it shoots Hornady Black just fine; ANYTHING ELSE not so much (I was under the impression that the .223 Wylde did a better job of balancing chamber pressure but that doesn't seem to matter)
- tried the upper on the bushmaster xm15 lower (no different)
-tried the bushmaster BCG in place of the fancy Aero NiBo (no different)
- tried six different types of mags -aluminum, steel (three versions of steel) and two different types of poly mags; I found that aluminum mags generally suck donkey balls.
- changed the gas block and tube using alignment pins (no change)
- tried a dozen or so permutations of three different buffer weights and two different buffer springs (no change)
- sent the upper to Aero and they evaluated the upper; nothing wrong with the upper receiver assembly
- put the CMMG .22LR conversion in and it works great.

I have a spare lower that I'll build out for the 18" upper.  I'm not giving up on it by any stretch.  I need to assemble a reliable blaster, for now, while I continue to tinker with the 18".
I do like the soft shooting 18".

I'm just throwing out some ideas as it looks like you've done a thorough job with troubleshooting and ruling out various component issues. What lube are you using and where is it applied (there was a recent thread regarding grease in the RE)? That barrel looks like it has a 0.0935" (#42) gas port so if you pull the gas block off again you could verify that (I understand that it's been back to Aero). If you test again, load one round in a mag and shoot after the malfunction occurs to check for bolt hold open, this may help verify it being undergassed. Check the condition of the brass for unusual ejector/extractor signs, it may point to a tight chamber. Also check the condition of the chamber, rough/dirty/oily chambers increase required cycling force.

Did you take weights OUT of the buffer?  Had to take all the weights out and use a LW carrier on my 27" pencil w/ rifle +3" gas it was so undergassed.

Wylde has little to do w/ gas - it shortens the leade to the rifling, making for a more accurate chamber than the 5.56 NATO spec.

16" should be mid-gas, so you can mount your bayonet properly, and have slightly less violent cycling than carbine gas.

Have you gauged the throat dimensions and chamber diameter to be sure everything was correctly reamed and is not too short, and/or too tight ?  Checked headspace ? Inspected chamber for excessive roughness ?    Any feed ramp issues ?

I did use headspace gages and it performed as of things were correct.
M4 ramps in the lower, m4 ramps in the barrel.
I didn't disassemble any buffer inserts or remove any weights. I have the Vltor A5H0 on the white spring now. I've tried also the green spring.
I'll try to figure out a place to post some images; I no longer participate in any social media nonsense, so that's out.

Could you be more specific about ammo?  Hornady Black comes in both .223 (lower pressure) and 5.56 (higher NATO pressure) versions.  Can you ask Aero what ammo they used for testing your upper?  Have you tried M193 or M855 LC 5.56?  Have you tried any other specifically marked 5.56 power ammo?

If its the higher pressure 5.56 version, you may, indeed need to shoot 5.56 ammo only, or open up the gas port.  I agree with an early post that tbe barrel port needs to be measured.

If the Hornady Black is the lower pressure .223, then your problem is baffling to me, given the diagnostics you have run and different solutions you have tried.  Have you tried M193 or M855 LC 5.56?  Have you tried any other specifically marked 5.56 power ammo?

Wylde chamber is intended to permit 5.56 pressure ammo use, and it does.  Difference is in throat and leade configuration.  Effect on port pressure is negligible.

Very easy to remove weights from the buffer w/ roll pin punches.

For what it’s worth, a rifle with a 16”, carbine gas barrel and an “H” buffer should eat any reputable .223 Remington or 5.56mm NATO round all day long without overworking anything - including the shooter’s shoulder.

The standard bolt carrier group acts as a metering system to control cycling.  It vents excess gas out those little holes on the right side (where the scooped out part is).  Having the system work this way allows it to run with a variety of pressure levels, from “mild” to “hot,” without operator intervention.  That’s a GOOD thing.  Cycling might feel “sharper” or “softer” with different loads, but the rifle should cycle at somewhere close to the same rate regardless.

Mid length is good for a couple of reasons.  First, by the time the bullet passes a mid port, the gas pressure has dropped some, but also the gas volume left to pressurize the system is less.  That makes it cycle “softer,” and usually means a lower cyclic rate (even semis have a cyclic rate - it’s how quickly the weapon fires, ejects, and reloads) than a carbine system with the same ammunition.

I used Springco MGL on the spring.
I have used a few different lubricating oils Hoppe's, Rem-oil and Shooter-Lube so far.

I finished a second build.  I've pretty-much isolated the problem to the VLTOR buffer system, when it is combined with a gas system longer than carbine-length and the RA434 trigger.

When I put my Bushmaster upper on the lower that I just described, it cycles the crappiest steel ammo just fine.

The 16" mid-length upper that I just assembled cycles properly on the Bushmaster lower; NOT on the VLTOR lower.

The 18" rifle-length upper that I originally had problems with cycles just fine with a lower built with the rifle-buffer and a mil-spec trigger. It would NOT cycle with the RA434 trigger and the rifle-buffer.

I may move away from the cartridge trigger and try musical-uppers again, just to see if it's the root of all the problems.

Thanks for your contributions.  At least I have THREE rifles that each work, and seemingly reliably at that (so-far).

The VLTOR A5 is not any different than the rifle lower.  The buffer just has the spacer cut off the front.  

I think it’s your BCGs.

This.
16" carbine, then use a tunable gas block. The Odin Works GB is the ticket for me. OW has GB that you tune and leave it, and an adjustable type that you jab with a tool to click it to go back and forth between suppressed and not.

The further out the gas port is the longer the dwell time to get the BCG going.

Narrower freebore.

Wait, what?  You got a diagram?

Freebore is longer but tighter.

Unsuppressed
10.5-12.5 carbine
13.7-16 mid
18+ rifle

Suppressed
10.5-11.5 carbine
11.5-16 mid
16+ rifle

Some overlap on the suppressed settings as both 11.5 mid and 16 rifle can run very well. But they have huge gas ports and less dwell

I didn't include intermediate, which is awesome. Or pistol which is retarded in 556

That's an AWESOME diagram.  Thank you for that.

I will second that above 16", use rifle gas, in 5.56x45mm, don't use longer than rifle gas, no matter how long the bbl is over 20".

I've also heard tell that if you tailor your load to a 7.5" bbl, such as 40 grn projectiles & loads w/ faster powders, 7.5" ballistics for 5.56x45mm can get quite sporty, as you have a lot more case capacity to play with than things like 5.7x28mm.

One thing to keep in mind with this drawing is that it applies ONLY to the listed reamers.

in general the GI 5.56mm chamber has a longer leade with a gentler rifling angle.  Unfortunately, I don’t have any specific data on exactly what the dimensions for the GI reamer are.

The SAAMI chamber dimensions are published in their drawings (page 68 of ANSI/SAAMI Z299.4 – 2015).  They call for the chamber to transition from 0.254 to 0.224” diameter in only 0.015”.  That’s not much length, and this is where “it’s not safe to fire 5.56mm ammo in a .223 Remington chamber” comes from.  Supposedly this should provide better accuracy in bolt action rifles.

But SAAMI’s standards are voluntary standards.  That means that Barrel Maker A might decide to make that a 0.03” transition, while Barrel Maker C might make it even longer.  It’s more about lawyer stuff than compliance with SAAMI standards.  But if you take those standards into account, you’re going to be safer.