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.