Anyone seen this? (ar-15 problem)

[nmxdaven]

Specs:
M&p-15
-carbine length gas system, 16" barrel running with a standard a2 buffer spring and buffer. (magpul PRS)
-m16 bolt assembly
-Stock everything else.

About every 5 or so rounds, when the gun is chambering a new round it will do this.

The round is severly bent at the bottleneck, and there will be several large gouges on the casing. It may take 2 rounds to do this, or sometimes i can get through 15 before it happens. Ive tried 8 diffrent GI- style magazines of all make.

Heres the strange thing... I tried one of my sig 556 magazines in it, and it functions flawlessly. But ONLY the sig 556 magazines. Standard GI, Pmags and H&k stainless mags all fail to function. The gun functioned flawlessly before its recent makover.

Any ideas guys?

 

[Jeff White]

The gun functioned flawlessly before its recent makover.

What did you do to the weapon?

 

[nmxdaven]

A m16 bolt assembly (From a colt ar-15), a magpul prs, and a freefloat tube.

 

[ny32182]

Assume that is happening somewhere outside of the chamber?

 

[taliv]

any chance you have a M4 feed ramps on your receiver but not on the barrel?

next time it happens, take a pic (or at least describe in detail) where the cartridge lands. is the bullet tip stuck on something? is the bolt/carrier coming over the cartridge?

 

[68wj]

any chance you have a M4 feed ramps on your receiver but not on the barrel?

My first thought too but I would think that the problem would have occured before too. OP, can you post a pic of your upper receiver/chamber area?

 

[F40]

Can you post a pic of your feedramps? And what was the recent makeover? Barrel replacement?

 

[nmxdaven]

I should have taken some pics when I was just out there, but ill describe it the best I can.

This is about where the bolt carrier gets stuck when it happens (Im just manualy holding it back in this pic)

Its quite a ways forward, so the round is in the chamber itself. Also, it can take quite a bit of force to extract the round with the charging handle.

taliv: The barrel and reciever are all stock m&p, so they should be m4 all the way around.

 

[nmxdaven]

Shot of the ramps, as good as i can get with this iphone camera.

 

[F40]

What ammo are you using?

 

[nmxdaven]

F40:

umc .223 brass. Its the only thing this gun has ever seen.

And the barrel was not changed. It only has about 2k rounds through

 

[mc223]

Looks like a magazine issue to me. Does not appear to be fully engaged into the lower and is lifting the cartridge nose on its rearward travel then pushing the cartridge out of the mag at the side of the cartridge. Then by velocity(of the BCG) bending the cartridge in the chamber mouth.
The M-16 bolt carrier is not heavy enough to do this alone but the use with an H2 buffer might be enough to exaggerate the mag issue. The use of the A2 buffer and spring would not seem to be of an issue.

 

[nmxdaven]

mc223:
You know my first thought was that it was getting too much force from the buffer, but the buffer and buffer spring are both stock from a a2 stock kit. it just seems weird to me that the stock buffer system would be generating that much force.

Also remember that it works great with my sig 556 magazines. And I have no frickin clue why that would be. They seem absolutly (functionaly) similar to any gi mag.

Any case, do you think I should try and take a coil or two off the buffer spring to soften it up a bit?

 

[mc223]

OOPs doubled on that one.

 

[Zerodefect]

Try some Federal .223. UMC is cursed. I don't know why, but all my guns (or is it my hands?) hate UMC.

Try a fresh Pmag as well. Deos the gun go back to working well if you put your old bolt carrier group back in?

 

[Sniper X]

measure the difference between the mag release hole in the mags in reference to the top of the mags. Sounds to me like it MIGHT be the Sig mag is allowing the sig mag to seat a couple mm deeper or so over the rest.

 

[mc223]

I would not be cutting springs.
Your lower may not allow the mags to seat up against the upper far enough to properly feed. At which time I would be contacting S&W.

After 5 or so round the spring tension of the magazine is reduced and perhaps allowing a bit of rearward tilt of the follower.
I know the P-Mags are the next big thing and all but they dont work well with my old Bushmaster so I just dont use them. I have a few C-Mags aluminum with antitilt followers that have never failed. I also have a couple that I got with the Bushmaster that have been very good.

My point is simply that Mags can and will be most of the problems with feeding you will encounter.

 

[HorseSoldier]

Heres the strange thing... I tried one of my sig 556 magazines in it, and it functions flawlessly. But ONLY the sig 556 magazines. Standard GI, Pmags and H&k stainless mags all fail to function. The gun functioned flawlessly before its recent makover.

Looks like your weapon is short stroking occasionally but not always. Very odd that the crappiest magazine you own is the one that works, though (well, the HKs may be crappier, but USGI and PMags should be good to go).

Did you make any adjustments to the buffer tube when installing the PRS stock?

 

[Jeff White]

I'm going to concur with HorseSoldier. I think you are getting the occasional short recoil. You may be losing gas at the port in the barrel or at the carrier key on your new M16 carrier. Are the screws that hold the key to the bolt carrier staked? Did the gas tube get slightly bent when you reinstalled it after putting the float tube on?

 

[TScott]

So after reading this forum daily for the past three weeks (lots of great info) I think I may be able to finally contribute something worthy as I have had almost the same exact problem with a Bushmaster using UMC ammo. Surprisingly I was using Sig magazines that didn't feed the round all the way up and caused a few casing to be bent and bullets to be jammed back into the case mouth, but once I picked up some PMags the problem disappeared (same result with stock mags).

 

[nmxdaven]

measure the difference between the mag release hole in the mags in reference to the top of the mags. Sounds to me like it MIGHT be the Sig mag is allowing the sig mag to seat a couple mm deeper or so over the rest.

You hit it right on the head. The Sig 556 Mag rest release hole (or whatever you call it) is SLIGHTLY above the others. ( GI, Pmag, Sig 556) Probly a mm or two.

Im still curious as to what this means and why it started. This is a rifle that would function with any clip you put in it before, and nothing in the lower was touched in any of the changes.

HorseSoldier
It was origonaly a carbine, so the tube and spring/buffer were replaced with stock a2 parts.

Jeff White
I actualy changed to the m16 colt bolt assembly because it was staked so good. The stock m&p carrier was pretty crappy with half assed staking. The front gas block was replaced with a yankee hill low pro (I used a rifle length freefloat) and the gas tube was fine. I wonder if any of this could be caused by using a carbine gas system to run a rifle buffer though?

TScott
Did you have that problem after changing something, or just from the factory?

 

[TonyAngel]

I'm thinking that the sig mag catch notches being higher is just relieving some of the tension of the next loaded round rubbing on the bolt while it cycles. It seems to me that you're having a sort of short stroking problem. When you went to the full auto carrier it slowed the system down enough to cause the problem to manifest itself, although it's probably always been there. You've just never seen the symptoms. What is your ejection pattern like? 1, 3 or 5 o'clock?

 

[nmxdaven]

I'm thinking that the sig mag catch notches being higher is just relieving some of the tension of the next loaded round rubbing on the bolt while it cycles. It seems to me that you're having a sort of short stroking problem. When you went to the full auto carrier it slowed the system down enough to cause the problem to manifest itself, although it's probably always been there. You've just never seen the symptoms. What is your ejection pattern like? 1, 3 or 5 o'clock?

now that you mentioned it... while I didnt pay attention to the pattern, I did notice much more rubbed off brass on my deflector than usual. Id say 5 o'clock.

 

[68wj]

Make sure your gas block and tube are installed correctly after the free float job. If you are leaking gas you will have short strokes.

 

[nmxdaven]

Alright guys, I just checked over the gas system again and its in perfect shape, so heres the plan.

With the ejection being around the 5 o'clock, I was thinking of taking two of the weights out of my stock rifle buffer and trying again. Does everyone think that will be a good ammount of weight off the buffer to help the cycling? I belive there are 4 weights stock, so that would be half the weight of a standard, putting it at around 3 oz... if that would put my brass in the right area id be happy. Good plan?