First time AR owner/builder

[cbrandsen]

I just finished building an AR15 in 350 legend. I must admit that I did buy the completed upper. Building my own seemed to be cost prohibitive but I can already tell some of the things I want to replace. Anyways, I put around 40 rounds through it and had multiple issues with the bolt not completely closing after chambering the next round. After ever 1 or 2 shots I drop the mag and and the bolt closes and fires fine. I completely stripped and cleaned the BCG and and lubricated all moving parts. Is there something I'm missing? Could it be the mag? Or is it likely the ammo?

[Moleman-]

AR's have a whole host of things that will make them run crappy. So you're saying the bolt doesn't close all the way and all you have to do is drop the 350L mag and the bolt closes by its self and will fire? Then I'd look for wear marks forming on the carrier and mag feed lips from rubbing on each other. If you can try a different 350L mag to see if that cures it or if the magwell catch slot was milled too high on the lower.

[pkuptruck]

if i read your issue right, 99% of the time on the 350l that issue is a tight extractor. take out your bolt, and remove the extractor
o ring, or donut..

some folks will say to hog oit the front of the magazine, but that is not the fix.

try it and let us know.

[Garymac]

Change magazines

[Scoop]

Change magazines

This. Always eliminate the easy first.

[cbrandsen]

I took it out this weekend and have 2 identical magazines. 1 ran flawless the other not so much

[joepistol]

If 1 mag works well in your rifle..the problem isn't the rifle.

If you haven't already mark, or # the mags, so you can tell them apart.
Compare the mag that works to the one that doesn't.. measure dimensions & see if you find a difference.
Mag lips identical ? other consideration may be follower, or spring.
Could even swap spring & follower from one mag body to the other, to see if it makes the inoperable mag work.

But First .. I'd contact the mag supplier or manufacturer & inform the of your experience, see what, if anything they have to say.