Bolt inconsistently locking back on 10.3" carbine

[Iraqgunz]

I would use a standard MILSPEC BCG and not an NiB one and see what happens.

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.

[bb223]

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.

That's nearly a starting load, isn't it?

With Ramshot Tac and 55gr projectiles, I don't ever go lower than 25gr.

Try bumping up the charge and see what happens.

[titsonritz]

Run some 5.56 ammo through it and see if the issue persists, if not there you go.

[MWAG19919]

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?

The pistol lower I built is the only one I can legally use with the mk18 upper. I have a BCM upper and my dad has a DD upper, both 16" middies, that have been 100% reliable. The BCM is on a BCM lower with carbine spring and H1, and the DD is on a PSA lower with an unmarked buffer and their spring, presumably both carbine parts.

That's nearly a starting load, isn't it?

With Ramshot Tac and 55gr projectiles, I don't ever go lower than 25gr.

Try bumping up the charge and see what happens.

It's about midrange for .223, but I just checked Ramshot's website and it is indeed a starting load for 5.56. I can go as high as 27.3
http://www.ramshot.com/wp-content/up...2016_Web-1.pdf

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.

Not partial to this load necessarily, but I am partial to handloading. I reload for around $0.12/round. Fortunately, it wouldn't add any significant cost to bump the charge weight up a grain or two.

Run some 5.56 ammo through it and see if the issue persists, if not there you go.

I test fired it with a round of Federal XM193 and it locked back with the DD magazine, but I realize that's too small a sample to give any relevant data on the ammo or magazine (though I suspect you guys are correct that a 5.56 load would lock back). I can try some more 5.56, but I gotta buy it first. I honestly only have the 5.56 on hand for a course in September. Otherwise, I handload for barely more than half the cost of steel cased ammo.

I can't afford to shoot a ton of factory 5.56, so I want this gun to function reliably with my handloads. 24.5 is convenient because I load on an XL650 and I have a 62 gr BTHP load that is sub MOA at this charge weight. It's a PITA to adjust the powder measure, but I'm more than willing to bump up the charge weight on the 55 gr load if that makes this gun happy.

[MWAG19919]

I would use a standard MILSPEC BCG and not an NiB one and see what happens.

I'm genuinely curious why you say that. The toolcraft was merely a spare I had lying around until this build happened, but I would expect a BCG with a lower friction coefficient to lock back before a phosphate coated one.

Not being argumentative, just curious.

[glockshooter]

I can't afford to shoot a ton of factory 5.56, so I want this gun to function reliably with my handloads. 24.5 is convenient because I load on an XL650 and I have a 62 gr BTHP load that is sub MOA at this charge weight. It's a PITA to adjust the powder measure, but I'm more than willing to bump up the charge weight on the 55 gr load if that makes this gun happy.

This may be the basis of your problem. If you are at the starting with Powder charge for .223 that it truly designed to be shot in a bolt gun. I certainly under the convenience side of what you’re saying but it sounds like your convenience is part of your problem. So in my opinion you have two choices. Either adjust your powder measure back and forth as needed or buy another tool head and powder measure you can switch. I’d be willing to bet you are causing your own problem. You could verify this but trying factory loaded ammo.

[MWAG19919]

This may be the basis of your problem. If you are at the starting with Powder charge for .223 that it truly designed to be shot in a bolt gun. I certainly under the convenience side of what you’re saying but it sounds like your convenience is part of your problem. So in my opinion you have two choices. Either adjust your powder measure back and forth as needed or buy another tool head and powder measure you can switch. I’d be willing to bet you are causing your own problem. You could verify this but trying factory loaded ammo.

Just to clarify, this is a midrange .223 load, but a minimum 5.56 load. For .223 Ramshot lists 23.2 — 25.8, and for 5.56 they list 24.5 — 27.3 for 55 gr Hornady bullets seated at the crimp groove (2.200"). But you're right my current charge weight is likely the culprit in this gun.
http://www.ramshot.com/wp-content/up...2016_Web-1.pdf

I'm definitely going to try two things.
First, I'm going to buy a box of 5.56 and see if it locks a variety of my magazines back. Hypothesis: it will work perfectly
Second, I'm going to work up my handloads at .5 gr intervals up to 27.0 (max for 5.56 is 27.3). Hypothesis: the problem will probably go away at 25.0 and almost certainly go away at any charge higher than that.

[MWAG19919]

I just got back from the range. Tested the gun with 5 different magazines (DD 32 round, Lancer L5AWM 30 round, Gen 3 PMAG 30 round, Gen 3 PMAG 20 round, and D&H 30 round). Loading one round into each magazine my dad and I shot 10 rounds of each ammo type.

Federal American Eagle 5.56 locked the bolt back 10/10 times
25.0 gr TAC locked the bolt back 4/10 times
25.5 gr TAC locked the bolt back 10/10 times
26.0 gr TAC locked the bolt back 10/10 times
26.5 gr TAC locked the bolt back 10/10 times
27.0 gr TAC locked the bolt back 10/10 times

Going forward I'll find a load between 26.0 and 27.0 that the 16" middies shoot accurately. That should give me plenty of wiggle room to find a decently accurate load that will still work when it's below freezing.

[joeg26er]

Great - sounds like your load was simply under gassed

[26 Inf]

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.

Not that I'm questioning what you say, buy could you tell me where I can find that info - I'm a copy and paste kind of guy. Thanks.