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?

[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.

[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.

[Iraqgunz]

NiB is garbage. In addition NiB changes the dimensions of the components.

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.

[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.