AR-15 feeding problem

[wchartz]

I have a S&W MP-15. It is feeds reliably except when using Magpul magazines and then only on the next to last cartridge. The bolt will start to strip the cartridge from the magazine then rides over the case. The bullet nose will be at the top of the feed ramp
and the bolt head will be resting on the case just below the shoulder. When I clear the jamb and release the bolt, the last cartridge will chamber normally.
Any suggestions on how to correct this?

 

[Trigger Monkey]

The easy answer is to don't use Magpul magazines. I know, obvious right but it will work.

That being said, how many Magpul magazines have you used in the rifle? Magazines aren't infallible and it appears there could be some tolerance stacking going on that's causing these issues. Without being able to see the lower and the magazine there's only so much advice to provide.

What other magazines have you tried?

 

[2aBaC̶a̶]

Doesnt sound like the mag is seated all the way.

In some cases if the bolt doesnt cycle all the way back the bottom of the Bcg will grab the round but since the bolt face is actually on top of the case as it slides forward it jams.

Adjustable gas block?

 

[Trigger Monkey]

How clean was the rifle? Was it dry or lubed? How lubed up was the rifle? What lube did you use?

Any modifications to the gun like the adjustable gas block mentioned above or different buffer springs or buffer weight?

Is the rifle over gassed? (shooting brass out to 1:30-2:00 position)

 

[WestDesertPRS]

How clean was the rifle? Was it dry or lubed? How lubed up was the rifle? What lube did you use?

Any modifications to the gun like the adjustable gas block mentioned above or different buffer springs or buffer weight?

Is the rifle over gassed? (shooting brass out to 1:30-2:00 position)

If this is happening with the same mag then I'm thinking a weak magazine spring isn't pushing up with enough force when there's only a couple rounds left.

When it's full it would be under plenty of tension. Empty maybe not enough tension.

 

[wchartz]

The rifle had about 200 rounds on it but this does not seem to have a pattern tied to round count. The rifle is dry lubed with Moly. It has the factory gas block and spring-buffer. The cases come out at 3-4 o'clock. It will feed with the supplied factory magazine. I have not tried other Magpul 20 round mags. My 30 round Magpul seems to work OK. I have not tried this but I wonder if I filed a small amount of material off of the magazine lips, would it help? I also thought about trying to stretch the spring a little but I doubt that this would help. The magazine probably has not been cycled more that 20 times.

 

[bfoosh006]

What Gen Magpuls ? Gen III with the anti over insertion tab on the back ? ( sometimes the tab can inhibit full seating )

Check to see if the "tab" is making to much contact... I have filed off some of the top side of the tab to help them fully seat.

And don't mess with the feed lips or mag spring yet.

 

[1911hombre]

If I'm reading this right, you're having trouble with a 20 round Magpul, correct? Is it straight body or curved? The earlier straight body mags weren't forgiving and honestly didn't feed reliably in many guns. Any Gen of the 30's feed best IME.

 

[wchartz]

It is straight body.

 

[1911hombre]

It is straight body.

I figured. They are finicky.

 

[Trigger Monkey]

Also, use a wet lube and try that

 

[Greg Langelius *]

This is a totally unofficial suggestion.

Sometimes, a mag spring gets tired, really wants to be a tad longer, and complains by wimping out when it's at its longest in the magazine.

I learned to humor my springs by taking them out of the mag for a bit of air, giving them a slight stretch, and putting them back into the mag.

Some of them have been grateful enough to push those last rounds out a bit better.

Remember, you didn't hear this from me; it's totally unofficial...

Greg

PS, I hate it when a girl mag gets her lips filed. It makes them cranky, and sometimes, they just give up the ghost. I say girl mags, 'cause guy mags can almost always get 'em up...

Again, I was never here, I never said nuffin', that's my story and I'm stickin' to it.

 

[101stinfantry]

Not being a smart ass, but D&H aluminum mil-spec. No self-respecting man wants a low cap 20 round mag, j/k, don't skewer me.

 

[WestDesertPRS]

Not being a smart ass, but D&H aluminum mil-spec. No self-respecting man wants a low cap 20 round mag, j/k, don't skewer me.

Well...to be fair the 30 rd mags get in my way when I'm hugging dirt on my bipod with my SPR...

 

[101stinfantry]

Fair enough, and I do that myself quite frequently. Do you, or can you source some aluminum straight 20 rounders?

 

[WestDesertPRS]

Fair enough, and I do that myself quite frequently. Do you, or can you source some aluminum straight 20 rounders?

I have a shitload of mags and a couple dozen are 20rds. A bunch of D&H aluminum 20ds and a handful of the straight 20 rd PMAGs

I was scarred by the 94 AWB when people started selling 17rd glock mags for $100 each at gun shows. So I never pass up an opportunity to throw another few in my stash.

 

[101stinfantry]

Me too! Though I would hazard a guess that it's the magpul mags that is the problem, somewhere in the lock up. Could be in the way the bolt is presented the cartridge, or might be in the mag catch, not letting the magazine insert far enough, or the mag catch slot not being milled high enough, etc, etc. However, I'm going to go with a mag problem being my first guess, always look at the easiest solution first!

 

[WestDesertPRS]

Me too! Though I would hazard a guess that it's the magpul mags that is the problem, somewhere in the lock up. Could be in the way the bolt is presented the cartridge, or might be in the mag catch, not letting the magazine insert far enough, or the mag catch slot not being milled high enough, etc, etc. However, I'm going to go with a mag problem being my first guess, always look at the easiest solution first!

Since mags are cheap and plentiful the 1st step is to try a different one and see if it exhibits the same issues. If not then chuck the bad mag.

 

[101stinfantry]

Yeah, that's where I would start. Luckily, it's hard to find good AR mags, that aren't cheap these days.

 

[YammyMonkey]

How old is your rifle? We have some of the original M&P T's at work and when I pulled them apart to unfuck them, I found most of them had loose/poorly staked gas keys, and all of the gas ports gauged (shockingly) on the smaller end of the spec. They also have aluminum gas blocks, which doesn't help matters. If yours is in the same boat, and trying to feed out of one of the older Magpul 20's it might be enough of a compounding of errors to cause that issue. So far, about a third of our rifles were un-fixable without actual gunsmith intervention, and the problem we had was the same as yours with a variety of 30 rounds mags.

 

[Greg Langelius *]

I use 20rd for anything I intend to shoot with a bipod.

All my 30's are loaded with M855, and are for my M4gery.

If my project to replace my M855 bullets with 62gr Speer Gold Dots is good to go; that's what will happen.

I've never had a mag failure with these.

Greg

 

[S197]

I had the same problem with the 20rd straight pmag. I cleaned up excess material inside of the lip. It works ok now but it is not in my mag rotation. I love 20rd mags but I buy older 223 colts, and D&H.

 

[Yondering]

Also, use a wet lube and try that

This.

What is it with all the new AR owners wanting to run their rifles dry lately?

 

[KaRwithaScar]

I have a S&W MP-15. It is feeds reliably except when using Magpul magazines and then only on the next to last cartridge. The bolt will start to strip the cartridge from the magazine then rides over the case. The bullet nose will be at the top of the feed ramp
and the bolt head will be resting on the case just below the shoulder. When I clear the jamb and release the bolt, the last cartridge will chamber normally.
Any suggestions on how to correct this?

What generation magpul mags are you using?

 

[wchartz]

And the winner is---------1911hombre! After an afternoon of testing different mags loading them with 3 rounds and shooting until dry, I find that the only mags that fail are the 20 round straight Magpuls. I had no failures with the OEM 20 round steels or the 30 round Magpuls. As an added bonus I could rationalize shooting about 100 rounds in the interest of scientific inquiry. Thanks to all who responded.