Odd AR pistol failure to feed

[Astocks2622]

I just finished my first AR pistol build in 300 BLK.
For troubleshooting purposes, here are the components.
Anderson lower with PSA lpk, PSA pistol buffer tube, buffer, and spring.
Aero Precision upper & bcg.
KAK 8.5" 300 BLK barrel, pistol gas, Anderson Manufacturing adjustable gas block, KAK Slimline flash can
ALG EMR V2 10" handguard

When I got it to the range, the first 30-40 rounds were flawless.
I was shooting factory magtec 125 gr supers, then switched to a bit of load development with 110 gr Barnes and Hornady. All rounds cycled properly, ejecting to about 4 o'clock.
Then I switched to some factory 200 gr subs for kicks, and they wouldn't cycle. I opened up the gas block several turns, but was still getting some short strokes that would eject, but not strip the next round from the mag, leaving an empty chamber.
After exhausting the 20 rounds of subs without any improvement in cycling, I set the gas block back to the original setting that ran the supers just fine.

When I went to shoot more of the same factory 125's, I started having serious failures to feed. The bolt would come back far enough to strip the next round, but the bullet would jump high and miss the chamber, jamming against the chamber face and the inside of the barrel extension. Because of the extreme angle, the rear of the cartridge would sometimes slip down off the bolt, allowing the bolt to override on top of the cartridge.
This also started happening with manual bolt release too. I'd lock the bolt to the rear to clear the malfunction, strip the mag, reinsert, FIRMLY, and hit the release. Same problem as I saw mid shot string.
So as far as I can tell, it's not a gas issue, it's a feeding issue. Any ideas?
Pics included for clarity. In this specific instance, the bolt didn't override the cartridge, but the bullet is jammed high on the chamber face.
Thanks for any help!

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[rickyrick]

Same magazine every time or did you try different magazines?

[Astocks2622]

Mostly same mag, a pmag. I did try one metal no name mag, and it improved slightly, only jamming every third round instead of every other.

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[Mobuck]

If it's a consistent jam from the same side of the mag(every other round) probably a feed lip issue.
If it's a brand new rifle, maybe you need to clean and re-lube.
FWIW
I use a "lightened" BCG when shooting the subs w/o a muffler to get consistent function.

[smee78]

I also think it sounds like a mag problem. I would clean everything good and pick up a few new mags or borrow some and give it a try again.

[Astocks2622]

Ok, I'll detail strip and deep clean, and try some if my other mags.

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[stagpanther]

Get just about any other metal magazine other than Pmags and try them out--I use brownell's because I can easily adjust their feedlips--and they are dirt cheap. I don't shoot subs in 300BO cause I don't have a can so I'm not sure whether a gas issue could be contributing (sometimes over-gas symptoms can appear to the same as under-gas ones). After cleaning and lubing everything and being sure your headspace and bolt to extension lock-up are not issues--I would insert a magazine and slowly drop the bolt in a controlled way so that you can see if the cartridge is cycling properly.

Are you getting brass shavings from your cases inside the receiver--kinda looks like it? While this does happen fairly regularly--I don't think this is entirely normal and may be due to various friction factors--not the least of which new components tend to have sharp edges. However, I've also found (from doing my own builds) that a very slight misalignment between the bolt and the extension can cause all kinds of issues--and in my experience brass shavings have always been present when that happened.

If your bullet meplats are smacking the chamber face and jamming (that's what the picture looks like to me) then probably the cartridge is getting stripped at a bad angle--could be the feed lips--but I have also seen magazines where the stacking of the cartridges can be affected by the internal width of the magazine. That's why I hate the magazine designs that curve towards the front of the feed lips--I avoid those like the plague nowadays.

[Chainsaw.]

Start with a good relubing.

[stagpanther]

One other thing that occurred to me when configuring an AR pistol--you really don't have a "whole lot of room" to play around with in terms of timing the ignition to the gas port so that the whole system cycles reliably with a wide range of propellants. And because the 300 BO starts off with a small charge to begin with, my feeling is that increases those limitations. Throw in a can and adjustable block--I wouldn't have the guts to venture into that territory myself.

One thing I have seen in a mis-timed 300 BO pistol--is that the case can be pitched back at the ejection port deflector so that it impacts the case mouth and even can ricochet back into the upper receiver--causing all kinds of trouble as you can imagine. Are you finding any extensive neck/mouth damage on you spent cases?

Start with a good relubing.

They never give me a second chance.

[Astocks2622]

It's not a gas adjustment issue. It jams even when initially loading the gun by slingshot or from a locked bolt...

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[stagpanther]

It's not a gas adjustment issue. It jams even when initially loading the gun by slingshot or from a locked bolt...

Did you SLOWLY let the bolt close on the cartridge and watch what happens? If I understand you correctly, you're saying even then the cartridge ends up being fed at a bad angle and smacks into the chamber face? Does the bolt carrier and/or the inside of the receiver show any unusual wear marks indicating a possibly problem with proper alignment inside the receiver?

I'm still guessing your issue is magazine related, though 300 BO is a cartridge that generally is trouble free in "ergonomics" of feeding into the chamber.

[redlightrich]

To me, that bolt looks very dry. Chainsaw said it. Disassemble, clean, lube it up and try again. I use Mobil 1. It is resistant to the heat as well as being very inexpensive ( in comparison to dedicated gun lube).

Also, metal mags seem less prone to spread the lips.

Until that bolt and upper "seat in" you need additional lube. Don't be afraid to over lube it. You can clean her up after she runs a bit.

I had the same issue, and lube was the answer.

Good luck

Rich

[rickyrick]

Does look pretty dry

[Astocks2622]

Ok, I put another 180 rounds through the gun today, with the bolt and upper heavily lubed with moly grease. Same problems. If the 180 rounds, I had about 15-20 failures to feed, jamming the round between the lower bolt face and the chamber face. This was using 3 different mags, including a pmag and 2 PSA metal mags. I did find that smacking the bottom of the mag often was enough to align the round and allow it to feed. I am DEFINITELY fully seating the mags. Several times I had the same malfunction consecutively without removing the mag.
About half of the malfunctions occurred on a fresh reload, whether just releasing a locked bolt, or by slingshot.

It's not gas, it's not lube, and I doubt it's mags still, as all three run fine in my 18" 223 AR.
Is it possible that the mag latch hole is a bit low on the upper? That seems highly unlikely, but the fact that smacking the mag sometimes clears the malfunction had got me confused.

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[stagpanther]

Do you have a spare 5.56 BCG you can swap out? I'd like to see pictures of your barrel extension inside the reciever--sounds like maybe your cartridges are possibly missing the feedramps and instead maybe pitched at a high angle by a lug?? if they got pitched up like that smacking the chamber face that would possibly explain the bolt "over-running" the case and jamming it. Take pictures of the bottom and sides of your BCG as well--sometimes a misalignment withing the upper can throw the path of the BCG--for example an improperly installed forward assist pawl can do this.

[Astocks2622]

So I finally made it back out to the range today. I verified I was still having the same issue across several magazines and ammunition selections.
I then swapped lowers to see if the problem followed the lower or the upper, and wouldn't you know, the problem stayed with the upper. It didn't matter which lower and magazine combination I tried, it was consistently malfunctioning. So finally I switched bolts between my uppers, and suddenly everything ran beautifully! I tried it on both lowers, with every magazine and ammo combination I could come up with, and it ate everything without complaint.
So for kicks and giggles I put the known "bad" bolt in my 18" 223 Wylde upper just to see if the problem continued across rifles. It didn't miss a beat!
I don't know what was going on, but I was able to just swap bolts between my 300 BLK pistol and my 223 rifle setup, and both are now 100%.
Wish I understood what was going on, but I'm not complaining!

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The

[stagpanther]

Sounds good--though I'd keep an eye on your brass to see if everything is really working well. Could be something as simple as the bolt and extension lugs need to wear a bit--or even the bolt extractor might "hang onto" a spent case a little too long when cycled. I have a blackout build which when I first put it together I had jamming issues occasionally with the cases--which are trimmed much shorter than the parent 5.56--rotating almost 180 degrees by the time it hit the shell deflector on the receiver port, causing the cases to impede the cycling process and occasionally bounce back into the receiver.

[HiBC]

I don't think I'd select moly grease to heavily lube the gun with.
You might try CLP.
I'm not there,I do not have your answer.
The AR pistol in 300 Blk is a really far cry from Stoner's M-16 in .223. Near all of Stoner's timing and balancing of a semi-auto cycle is not present.

Re-engineering and finding a window of reliability ...its a wonder it works at all.

So,call it an adventure.

Trying all your loads,no telling what powder fouling may be in one BCG,not the other. Brass scrapings? Ejector plunger,extractor.I see a LOT of unburned powder.
Is there a M-4 feed ramp match issue?
A short buffer tube,etc leaves less time for a mag to feed a round up.

The 30 cal300 blackout bullet has a larger dia than the .5,56 bullet . So,when the bullet hits the feed ramp ,a .308 bullet radius is .154. a .224 bullet radius is .112 .So,the round is lifted .042 farther in the same amount of time/distance.Will .308 bullet and neck track the .224 bullet feed ramp troughs?Those might need some judicious re-contouring with maybe a larger dia stone than you'd use for a 5.56?I don't know.
How do the mag lips like the extra lift?
If it really is about one BCG vs another,WHY? Is one bolt face easier for the case head to rise up? Sharp edge digging brass?Some barrel extensions were not deburred/corner broke. The bite,dig. Amazing how much better even a.002 corner break is than a sharp edge
Is that larger dia bullet finding a sharp edge to snag in the barrel extension on top?
IMO,it will take that sort of understanding and attention to detail.

Buffer weight? Maybe(IDK) slower is better. I don't know if an Enodyne type hydraulic buffer of the right length is available,but it might induce some dwell.

Remember,it was originally a 20 in 5.56 design with a rifle length buffer tube.. Expect challenges.

[stagpanther]

Great stuff HiBC for "in-depth" analysis--I too would at be curious why it works in one in not another--but my intuition says gas system difference would be the most likely explanation between the two rigs--meaning the wylde "closer to normal" and increased/better timed pressure probably overcomes whatever issue there is with that bolt in the pistol. Because of the increased diameter at the case mouth and the "come-up" to the shoulder quickly I've found that cartridge lift on the ramps generally is good for the 300 BLK--certainly better than something like the grendel, for instance. Sometimes it's easy to think "problem over" just because everything works fine with the rifle--but I'm a believer in "The brass tells no lies" and generally it will give pointers to where issues may lie.

[brycewise]

Several things to check....
What is the OAL length of the rounds that wouldn't feed vs the ones that do?
Was your ejection pattern the same with the 125 when it failed to go into battery as when they did?
If you fiddled with the gas system... did you set it to allow for subs to feed reliably? If you did then that setting may come at the expense of allowing supers to feed reliably and the weapon is cycling to fast.

If none of these cure the problem... what is your preferred load and how does that feed? If its just 1 type of ammo that malfunctions and it isn't the one you prefer just make sure the one you do prefer cycles and stick with that.