Bolt inconsistently locking back on 10.3" carbine

[MWAG19919]

I just got back from taking my Mk 18 pistol to the range for the first time. I was getting an occasional failure to lock back with my handloads. This occurred with a 10 Gen M3 PMAG, a 30 round Gen M3 PMAG, a 30 round D&H mag w/magpul follower, and did not occur on a DD 32 round magazine. I fired about 100 rounds through the pistol today, mostly using the 10 round magazine. None of the magazines were new, and all have functioned perfectly in other ARs. The failure to lock back only occurred about half the time, so it's possible it would have occurred with the DD magazine if I had fired more ammo with it.

Ejection seemed weak, but spent casings were ejecting a few feet away at 4:00.

AMMO:
LC 16 brass
CCI 41 primers
24.5 gr TAC
Hornady 55 gr SP
2.200 OAL.

GUN:
Daniel Defense 10.3" Mk 18 upper with 0.070" gas port (have not and will not disassemble the upper to verify; Brownell's and DD both list this spec)
Toolcraft NiB BCG with BCM enhanced extractor spring, insert, and o-ring
SIONICS H2 buffer
Springco Blue spring


My hypothesis is that the buffer spring is the culprit, and switching to a Springco White or White Hot spring will cure this issue. I figured I'd post here and see what you guys & gals think before I start buying springs.

[26 Inf]

Here is what I'd do:

Make sure everything is sealed up correctly - gas block, gas tube, carrier key. Remember - it isn't completely abnormal for there to be some soot around the gas tube/gas block juncture.

I generally test fire a new build in stock configuration. In this case, a carbine buffer and regular carbine spring. That way I'm not chasing stuff.

So I'd try with a carbine buffer, carbine spring first; if that works then I'd try an try an H with the regular carbine spring (or the H2 if you don't have an H). If that works then I'd diddle with the blue spring.

JM .02 - probably overvalued.

[Bob Sacamano]

Would just clean, lube, then shoot some more. Will probably run like a champ once broken in a bit.

[joeg26er]

What are the Lower build specs?

[MWAG19919]

What are the Lower build specs?

Centurion CM4 stripped lower
SIONICS Builder’s LPK (no trigger)
BCM mil spec trigger (not the PNT)
Generic end plate
Generic castle nut torqued to 39 ft lbs & staked
H2 buffer
Blue Springco spring
SBA3 brace w/included receiver extension

[MistWolf]

Before doing anything, get some factory 5.56 spec ammo and start with a lock back check.

Stock configuration for an AR does NOT include carbine weight buffers. Companies seeking to cut corners and save money use carbine weight buffers because they do not use tungsten weights. A properly gassed AR should run with an H2 buffer and Sprinco blue spring with 5.56 spec ammo.

Do you have another lower you know works? Do you have an upper you know that works you can test your pistol lower with?

[joeg26er]

FWIW - I am getting some instances of the bolt carrier getting "stuck" inside the buffer tube on my newly installed SBA3. It's likely due to tolerance stack but that is something else that would contribute to cycling issues.

[26 Inf]

Before doing anything, get some factory 5.56 spec ammo and start with a lock back check.

Stock configuration for an AR does NOT include carbine weight buffers. Companies seeking to cut corners and save money use carbine weight buffers because they do not use tungsten weights. A properly gassed AR should run with an H2 buffer and Sprinco blue spring with 5.56 spec ammo.

Do you have another lower you know works? Do you have an upper you know that works you can test your pistol lower with?

Granted, most rifles will work with the H or H2 buffer because many companies punch their ports larger to ensure the rifle works with a wide variety of ammunition. But I've been told the mil-spec is a .063 port and a carbine buffer. If I'm I working on faulty info could you point me to a source where I can plus up? Thanks.

If I did not plan on shooting 5.56 out of the rifle, I would just as soon make sure it functions with what I'm going to shoot out of it.

The firearm needs to function with what you feed it. In the OP's case apparently he handloads, if he has a load he is partial to, I'd try to get the rifle (pistol in this case) to run on what he is going to feed it.

If going to a carbine buffer solves the problem, do you think that putting spec 5.56 through it will lead to a situation where the bolt is outrunning the mag spring, etc.?

[MistWolf]

When Colt was developing the M4, they first tried the carbine buffer with 3 steel weights and it gave them fits. When carrier speed were sufficient to ensure reliable function, they got bolt bounce. To stop the bolt bounce, they had to increase buffer mass by replacing one steel weight with one made of tungsten. The M4 was originally designed to be a PDW to replace the M1 Carbine, not a primary infantry rifle. The minor flaws the early M4s had didn't show up until special forces units put the M4 hard use in combat. It was discovered that while the H buffer worked fine during semi auto use and occasional burst or full auto use, heavy useage showed that there still problems with bolt bounce, particularly with the heavier SOCOM barrel. Colt again increased buffer mass by using two tungsten and one steel weight giving us the H2 buffer. This information can be found in the book Black Rifle II by Chris Bartocchi.

I have verified for myself that the AR runs smoother with H and H2 buffers than the lighter carbine buffer. In fact, testing has me convinced that the carbine buffer is simply too light even when the AR is properly gassed. Using an upper with an SLR adjustable gas block, I tuned the upper until the bolt would eject and just lock back on an H buffer. I then tried a carbine buffer. Recoil was sharp as the buffer banged against the back of the RE. I closed the gas by one click. Now, the AR would eject, but not lock back. I opened the gas block one click and the AR once again ejected and locked back. I then tried it with the H2 buffer. The AR ejected and locked back using the same setting.

During this test, I noted that recoil with the carbine weight buffer was sharp. The buffer was bottoming out hard against the RE. Recoil was softer with the H buffer and softer still with the H2. I also tried the the A5 RE and an A5H2 buffer and Sprinco green spring. The A5H2 buffer is heavier than an H2 as it's the same weight as a rifle buffer. Recoil was softer still. As recoil is too sharp with the lightweight carbine buffer even with the gas adjusted to a minimum setting, I can come to no other conclusion that the carbine buffer is too light.

However, I felt that I had reached the point of diminishing returns with the H2 buffer with a carbine length spring and the A5H2 and rifle buffer using a rifle spring. If an AR is still over gassed with an H2, A5H2 or rifle buffer, it's time to address gas flow. Going any heavier will result in the reciprocating mass pushing the AR around excessively.

The first rule tuning an AR (or any machine), is to start from the base line to start from. In this case, the base line is an AR tuned to run on 5.56 spec ammo such as the Colt 6920. By starting off trying to get an AR to run on downloaded ammo, another variable is introduced, complicating troubleshooting. Once it's established that the AR will run on 5.56 with an H buffer and standard carbine spring, then the shooter can down load the ammo and make adjustments for reliable function.

A reliable AR is a balance of gas drive, reciprocating mass and spring rate. The gas drive is a balance of gas pressure, flow and volume. Changing the ammo changes all three. While the AR can be tuned to run on lower pressure ammo, doing so will give it a narrower span of operation, limiting the utility of the AR.

MWAG has a 10.5" barrel with a .070" gas port, an H2 buffer and Sprinco blue spring. That should work with 5.56 spec ammo. Once it's established whether or not the AR will run with baseline ammo, we can begin figuring out just where the problems lies.

[Iraqgunz]

Incorrect. The MILSPEC is a .063 gas port with an H buffer. New carbines are being shipped with the SOCOM barrel and use an H2 buffer with the same gas port.

Granted, most rifles will work with the H or H2 buffer because many companies punch their ports larger to ensure the rifle works with a wide variety of ammunition. But I've been told the mil-spec is a .063 port and a carbine buffer. If I'm I working on faulty info could you point me to a source where I can plus up? Thanks.

If I did not plan on shooting 5.56 out of the rifle, I would just as soon make sure it functions with what I'm going to shoot out of it.

The firearm needs to function with what you feed it. In the OP's case apparently he handloads, if he has a load he is partial to, I'd try to get the rifle (pistol in this case) to run on what he is going to feed it.

If going to a carbine buffer solves the problem, do you think that putting spec 5.56 through it will lead to a situation where the bolt is outrunning the mag spring, etc.?