Got in some Remington UMC 220 subs this week and tested a couple boxes yesterday evening.
3 out of 40 rounds failed to fire. See pic 1 for primer strikes. All three rounds fired fine the second try.
I've been through roughly 260 yellow box Rem subs, 60 Gunn subs and numerous supers with no light strike problems. This lower sees most use with a 556 upper, and has never exhibited this behavior before. I'm confident the trigger group is not the issue. I'm also fairly certain the bolt/barrel/firing pin are ok as well since the other 750ish rounds have not seen light strikes.
Anyone have any ideas?
Has anyone had trouble with the UMC subs?
Those light primer strikes are too light to be a hard primer or a weak hammer spring.
Your issue appears to be with the bolt not going fully into battery or a head spacing issue.
This could be caused by the ammunition, but you would need to measure the failed cartridges or use a chamber gauge to make that determination.
Other things to check are your head spacing and bolt lockup.
Head spacing is most easily checked using a go / no go gauge, but can also be measured using a piece of brass and a micrometer.
Bolt lockup can be checked by riding the charging handle while slowly letting the BCG go into battery with an empty chamber.
A clean well lubricated BCG should go into battery without much force.
"Don't tell fish stories where the people know you; but particularly,
don't tell them where they know the fish."
--Mark Twain
Assuming you don't have a lightened hammer spring (some competition trigger groups do on purpose, and I am rabidly against this "game gun" nonsense). Check the following:
1. Firing pin protrusion - make sure it is within mil specs.
2. Chamber headspace - make sure it is within SAAMI specs.
3. See if unfired examples of the same ammunition are short on headspace.
Thanks to you both for the replies! To answer some questions:
Geissele SSA FCG. Everything there seems fine.
Firing pin protrusion is .030
Riding the CH forward, dirty but wet BCG, the bolt locks up easily every time.
Unfortunately, I don't have go/nogos for this caliber, so I can't check the headspace currently. Anyone have a set I could borrow?
Any other suggestions? I'm still leaning toward ammo, but I will have to shoot more so have a sample offender.
I actually had the same thing happen with 2 of the UMCs and 2 Ozark Ordnance 147s; light primer strikes at first, then they shot fine after I cycled them through again.
The conclusion I came to was my bolt was VERY dirty and the firing pin was not impacting the primer hard enough to make it fire. In fact, right after mine became finiky with the soft-strikes, my upper was so dirty that it failed to want to function anymore (feed rounds, fire...). Granted, since my last cleaning, I had about 400 FMJs unsuppressed and 400 or so subsonics suppressed...
I have a 9" AAC upper...
This would be my guess if you are shooting suppressed...
I ran into the same issues with UMC using a simple Handi Rifle. 1 or 2 misfires from every box. All fired on the second strike. No such issues with the better quality Remington green/yellow boxes.
