AR troubles.

[twoclones]

I bought a used Smith and Wesson Model M&P15T Rifle [AR-15 style] which I was told had about 500 rounds through it. It looks only lightly used... Today I took it to the range for my first time and had some trouble which I need some help with.

Of the first 30 round magazine, all fired and ejected without a problem. After the first 3 or 4 fired from the second magazine it failed to fire and the cartridge would not eject. When I opened the weapon, the cartridge fell out. The projectile was scratched and primer untouched. After 3 successive failures, I packed up and went home.

I didn't find any obvious problem when I cleaned the gun.

What might have gone wrong or what did I do wrong?

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[Logical1]

did you try to reload the first magazine and see if they would all fire out of that magazine a second time?

 

[Vstromvoyager]

Did you use the same mag each time?

 

[drand]

Were all the failures from the same magazine? It sounds like you may have a problem with that magazine.

 

[twoclones]

The second magazine worked for a few shots so I took it out and put it back in a few times. No luck. Being my first time using this AR I was afraid I was doing something stupid and left before thinking to try another mag. I have inspected it and can't find any damage or difference between it and the one that worked flawlessly.

It's a plastic Magpul Pmag, 30 round.

 

[JohnH]

Earlier production P-Mags DO NOT WORK with S&W AR's.
The mag well is ever so slightly different.
I am not sure if Smith has changed this on recent AR's or not.

The newer P-Mags will fit the S&W AR.

 

[MountainBear]

A failure to fire would not generally be caused by a magazine. If the rifle chambered the round but did not discharge when you pulled the trigger, check your firing components. I would start with the firing pin and move on to the bolt (including the extractor and ejector), hammer, trigger, and safety.
A faulty magazine will cause failure to feed, not failure to fire.

 

[JohnH]

I wasn't trying to say the PMAG was the problem for everything, just that is is a know problem for S&W AR's and not locking in.

My buddy has a S&W AR and had the same problem with FTF. We tried several things before we noticed the trigger pin was loose and worked it's way over and allowed the trigger not to function correctly.

 

[elsie]

...After the first 3 or 4 fired from the second magazine it failed to fire and the cartridge would not eject. When I opened the weapon, the cartridge fell out. The projectile was scratched and primer untouched. After 3 successive failures, I packed up and went home.

I'm guessing somewhat from the description above but what it sounds like is that the follower got stuck and did not lift the cartridge far enough to be stripped from the magazine. Without a more detailed description or picture of the cartridge you mention it is difficult to say. You say the projectile is scratched, but is it the bullet or the larger diameter portion of the case that is scratched? Is it only scratched on one side in a fairly narrow zone? IE, it was lying on top of the magazine and the bolt scratched it as it rode over. If the bullet is scratched, it is possible the point jammed as it tilted up to feed into the chamber, but I think in that case, the bolt would not fully close, which should be noticeable. Though I'm having trouble thinking of a particular circumstance which would leave the round loose in the upper after you dropped the magazine.

elsie

 

[eldbillbo]

I wasn't trying to say the PMAG was the problem for everything, just that is is a know problem for S&W AR's and not locking in.

My buddy has a S&W AR and had the same problem with FTF. We tried several things before we noticed the trigger pin was loose and worked it's way over and allowed the trigger not to function correctly.

Its the pmags problem i trade you a some old school GI mags for the pmags and solve the problem

stange problem that your able to pull the trigger and the hammer falls and no bang or dent in the primer

what kind of ammo were you using? did you clean the barrel and lube your carrier and inner buffer tube before shooting it?

 

[NWPilgrim]

I agree with JohnH. Sounds like the mag was not seated completely. A couple of rounds fed then the mag slipped down and the cartridge never made it into the chamber. The bolt slid over the top of the cartridge, thus scraping it.

As JohnH says, the early PMag and S&W combo is known not to work.

Try it with some milspec aluminum mags, and a newer PMag. I don't think there is anything wrong with your AR or your mags per se, just the older PMags are not good for the Smith. Perhaps you can trade your older PMag with someone that has another brand AR and newer PMags.

I have DPMS and RRA lowers and they seems to work with all PMags.

 

[PDXGS]

A failure to fire would not generally be caused by a magazine. If the rifle chambered the round but did not discharge when you pulled the trigger, check your firing components. I would start with the firing pin and move on to the bolt (including the extractor and ejector), hammer, trigger, and safety.
A faulty magazine will cause failure to feed, not failure to fire.

Agreed, but start with a new mags first and then take the bolt apart and inspect& clean it? There are several videos on YouTube.com from Brownells and the like that describe and show this process in detail.
J

 

[motoman98]

Big question and no one asked it: Did the rounds in question get chambered in the rifle? ... when it did not fire.
Big difference in "failure to fire" if the round is chambered or not!

 

[MountainBear]

Big question and no one asked it: Did the rounds in question get chambered in the rifle? ... when it did not fire.
Big difference in "failure to fire" if the round is chambered or not!

I assumed that since the OP went to the trouble of mentioning that the primers were not dented, then that meant the cartridge was chambered and simply did not discharge when the trigger was pulled.

 

[ktd]

Yup saying "the round fell out" is not descriptive enough. If it fell out of the chamber vs it fell out of the action area through the magwell or ejection port makes a big difference in diagnosing.

I had problem with light strikes in my issue FN. The drill pulled it apart, and the hammer spring looked like spaghetti, it had all sorts of bends it was not supposed to. Apparently some previous kid had taken it apart and had problems putting it back together and mangled the spring in the process.

Does not hurt to try new mags, though to me I think it is a bolt assembly issue somewhere. Possibilities: sludge in bolt slowing down firing pin, foreign matter somewhere else in the action (apparently one of the new fashionable accessories is to put a little plate over trigger group so that when you blow out a primer, it does not fall into the trigger), firing pin too short, super hard cci primers (that ones a joke ), out of spec hammer, out of spec spring, bad head space or other chamber issue, bad bolt head. I would try a good internal cleaning to start, especially the firing pin channel. I have fixed a lot of things with just a good cleaning.

 

[twoclones]

Trying to answer a few questions here...
The ammor is PMC Bronze 55 grain.
The bullet was scratched, not the brass case.
The round did chamber but would not extract so I'm thinking the bolt did not go 100% forward.
The Pmag that worked has a window but the one that failed has none.
Firing pin, extractor, bolt all appear in good condition.
Magazine being seated was the first think I checked and it was NOT seated properly the first failure. Appeared to be for the second and third failure.

Is there a way to identify which mags are the older style?

 

[JohnH]

Is there a way to identify which mags are the older style?

The windowed magazine should be a newer model.

The problem with the older model is the rib on the PMAG that contacts the receiver magazine well and prevents magazine from ‘locking' in.

You could insert the windowed magazine and observe the clearances in this area and then insert the non-windowed. You should be able to see where the magazine contacts the receiver and prevents lock engaging.

My buddy who bought an S&W 15T a while back and then bought a bunch of PMAGs only to find out they were not compatible. Lucky for him he owns his own machines shop and was able to run a ball end mill around the magazine ever so slightly to make them work.

There might be some PMAG experts out there that can tell you a better way to do this.

 

[ktd]

That was a common issue with the Thermold mags, and cutting back the mid rib a bit was the only solution I am aware of. When you put the mag in with the bolt locked back, it should jiggle very slightly, as opposed to being rock solid, which usually means something is binding.

 

[twoclones]

The windowed magazine should be a newer model.

Sure is. I found the manufacture date, perhaps of the injection mold?, on the magazines. All 4 of my non-window mags have the date 5/08 and the windows mag has the date 7/08.

Still researching to find when Magpul changed their dimension, clearance or whatever it was.

 

[ORBrit]

I'm sure you could switch for a newer PMAG with someone with another brand AR which works with older PMAGs.