Anyone seen this? (ar-15 problem)

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.

http://img257.imageshack.us/img257/8893/img0747i.jpg

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?

The gun functioned flawlessly before its recent makover.

What did you do to the weapon?

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

Assume that is happening somewhere outside of the chamber?

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?

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?

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

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)

http://img143.imageshack.us/img143/9163/img0751fi.jpg

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.

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

http://img249.imageshack.us/img249/3309/img0755mz.jpg

What ammo are you using?

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

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.

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?

OOPs doubled on that one.

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?

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.

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.

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?

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?

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).

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.

http://img51.imageshack.us/img51/2862/img0759w.jpg


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?

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?

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.

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.

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?

5:00 ejection sounds like it's might be right on the edge of being under gassed. Why don't you just go back to the standard AR15 bolt carrier.

How did you check the gas system? If your system is bleeding gas off someplace, I'd check the gas rings. I've had gas rings wear out in as little as 500 rounds.

Just don't go monkeying around, like taking weights out of the buffer, trying to cure a symptom. Find the problem. Although I suggested going back to the lighter carrier, your rifle should run with the M16 carrier as well. My guess is that you're bleeding gas someplace.

Without having the rifle in my hands it's hard to say where the leak is. It could be a bad seal between the barrel and block, block and gas tube, gas tube and gas key, gas rings or a combination of all.

What changed is the first question you should ask yourself.....Put the original BCG in and see if you still have the problem.

What changed is the first question you should ask yourself.....Put the original BCG in and see if you still have the problem.
+1

Try the original BCG you had in the rifle. Is the new gas block port matched to the barrel gas port perfectly?

It's either gas rings or the buffer/spring.

I'm with kaferhaus on the recoil spring and buffer.

carbine length gas system, (snip) with a standard a2 buffer spring and buffer. (magpul PRS)


Seems to me that if you are running an A2 buffer and recoil spring with a carbine length gas system, you'd be "undersprung". I'm no expert, but I think that would display the same symptoms as undergassed.

When you put the gas block on, did you account for the space where the former handgaurd end cap rested? The small area between the shelf on the barrel and the rear of the gas block.

The gas block might be a hair too far back.

I had that issue before I switched to a heavier buffer to slow down the BCG. It is ejecting ok?

The gas block, origonaly a hex screw on, was milled out to accept pins, so the gas block should be spot on. Ive done it a few times before with other rifles, and never had any probems. I checked it again yesterday (Pulled out the pins and checked the allignment, and it seemed perfect.

I dont particuarly want to go back to the standard carrier, as its pretty crappy. If I could get it running with this one I would be much happier. (and save a bit of money) But next time I go out I will try it with the origonal and see if that clears up my issues.

A m-16 carrier/rifle buffer should be working fine with a carbine gas system now that I think things through. Carbines run so gas heavy I shoudnt be having these issues at all. I was just VERY carfull about getting the gas system alligned correctly, so if it is a gas problem I would think it indeed might be the rings.

Al Thompson: Can you explain "undersprung" in a little more detail to me? I would think If anything I would be oversprung with the extra gas pressure compared to a rifle length, but I've never run diffrent gas systems with diffrent buffer systems before, so this is all new to me.

As I understand it (and I might be all wet), the carbine used a heavier buffer to add momentum to slow the cycle down. I'm thinking that with the A2 set up, you may be seeing the action cycle faster.

I've got a few minutes, let's see what I can find... :)

Edit:

Take a look at the first post (Mr. Roberts's post) and see what you think. I like to try the cheap solutions first. :D

http://www.thehighroad.org/showthread.php?t=199626

I suppose that it's possible, but I have never seen bolt bounce (too much gas causing the bolt to move faster then the spring in the magazine can feed the rounds) in a semi auto before. I have seen it in full autos, which is why the heavier buffers were developed for the M4A1.

I'm leaning towards a gas problem somewhere in the system. The fact that it ran fine until you changed it over eliminates the magazine issue in my mind.

I think I would try working backwards from what you changed starting with the BCG. That will at least tell you where the problem is. My guess is gas key or gas rings on the new BCG. Have you tried extending the bolt and standing the BCG up on the bolt face? If the gas rings are good, the bolt should not collapse at all.