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Have a unique, repetitive failure on an AR15 that I have not seen before and have no idea how to shade tree a repair.

DelTon lower, 556, less than 500 rounds through it.

Dissipator pattern upper, same age, pretty sure it is also DelTon but I forget for sure and am not finding markings immediately.

On a 30 stack, rounds 1,3,5 and sometimes 7 from the left side of the magazine are not stripping into the chamber but are jamming in the locking assemblage for the bolt face. If I pull the charger on good ammo, the bolt will drive the round home. On cheap ammo it will drive the slug into the brass and distend the throat enough to jam the bolt closed.

After getting through these rounds, the system feeds and functions normally

This is repeated in a 10 stack on round 1 only. Repeated regardless of magazine.

Any AR guys can point me at a possible why and fix I would appreciate it. Unfortunately Fergusons BS is preventing me from simply replacing it.
 
Your feed ramp has a burr or damage and or isn't mil spec.
Yup, you may need to polish up your feed ramps, also….

Check the feed ramps to see if it has the M4 feed ramp cuts in both the the barrel extension and the upper receiver where they meet up.

You can get away with a barrel extension that has the M4 cuts and a receiver that doesn't, but not the other way around.
 
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If it's a feed ramp issue why is it only happening towards top of mag count?
On the left hand side… could be the mag spring tension at those positions provide the "perfect storm".

Also check your bolt face for anything, and make sure the cam timing isn't out of spec…. maybe try a different bolt carrier with the Dissipator's bolt, the carrier could also be out of spec.
 
I have had issues with Mag-Pul Mags similar to this, would usually cycle two to three rounds just fine, then double feed and jamb up the bolt requiring a flip to dig out the stuck rounds, usually three, and then a re charge of the bolt to get it back in battery and firing! Some times this would happen through a full mag, sometimes it ran the entire mag with out issue, I even made a not of WHICH mags had issues, but it didn't seem to matter, it was random with all mags! What was crazy, it would run just fine with Mil-Surp mags just fine, and it ran Lancers with zero failures, and I have never seen this issue with any Mag-Pul mags until now! Checked my M-4 feed ramps and they are all within spec, but went ahead and polished them up super shiny and de horned the bottom edges, and that seemed to clear the issues entirely! I was convinced I had issues with the Mags, but I also knew there were known issues with "Mil-Spec" M-4 Feed Ramps not being true or having burrs or other issues, so I started there, and that solved the issues!
 
If it's a feed ramp issue why is it only happening towards top of mag count?
Because the extra spring pressure keeps the cartridge from tilting to follow the feed ramp. I had the same problem in a bolt action Howa long action receiver. Weakening the magazine spring slightly fixed the problem.
 
Depending on the mag brand, arent AR mags notorious for needing to be downloaded to function?
 
Depending on the mag brand, arent AR mags notorious for needing to be downloaded to function?
No, they are not overall notorious for that.

Early GI 20rd magazines did have this requirement; they are generally loaded to 18 rounds. Despite this, they are still prized as reliable and highly functional magazines and fetch premium prices used for even 50 year old magazines.

GI 30 and other brands are not inherently known to need downloading.
 
30 rds = 28, 20 rds = 18. Sharpie everything up, try a different mag (left side feed lip might be AFU), run wet and see were those left side rounds are getting hung up. Check for burrs, check/lightly polish M4 feed ramps.

Is upper cut for extended M4 feedramps?
 
Depending on the mag brand, arent AR mags notorious for needing to be downloaded to function?
Semi decent quality mags... that hasn't been my experience. More common is sometimes a full 30rnd mag may have difficulty seating on a closed bolt (typically newer mags where the springs haven't quiet "settled" yet), but feed issues... typically boils down to a ramp issue and easily remedied.
 

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