9mm CMMG dedicated lower with converted Uzi mags. FTF last 2-3 rounds. Every mag. Every time. I am using a standard carbine buffer spring. a 5.4 oz 9mm buffer.

Any thoughts?

Converted uzi mags, nuff said.

Probably be better off running Metalform instead of the Uzi's.

Thoughts:

Old surplus uzi mags? Take them apart and give them a thorough inspection and cleaning.  Weak springs may be the issue, or they may need a good cleaning and dry lube.  Since it's only the last few rounds, I'd suspect the springs.

I'd also check to make sure the mag catch isn't interfering with the followers.

Using a heavier buffer may slow the action a bit, giving the mags more time to get the rounds into position for feeding before the bolt comes back into battery.

Try Metalform or ASC mags.  If the problem persists, you may need to send your barrel to Rudy @ macon armory for his "guaranteed to feed" treatment.  Well worth it imho.

Is the bolt not stripping the round off the mag, or are the rounds being stripped but not chambering?

My converted UZI 25round mags do the same thing.
All my metalform work, so it clearly isn't the gun.

Had the same issue with Metalform mags, last two rounds. No issue with Asc.

Likely the mag spring doesn't have enough tension to present the nose of the cartridge at an appropriate angle when the bolt picks it up.  Macon Armory feed cone might help.  Do you have a pic of the feed cone of your barrel?

I know it has already been mentioned, a bunch, but metalform mags are what you seek to cure the feeding issue.

But in the meantime, you can disassemble a few of those uzi mags, clean the interior, verify no burrs on the mag catch cut,
verify the follower is smooth, and spray a little dry lube in there.
You can stretch the spring out a little before you re-assemble the mag, it's already worn-out anyway, and won't last  more then a few loads.
But it will be an easy way to verify the mag is the issue.




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