From another forum: M&P 15 Carbine woes.

[Bore Rider]

I originally posted my problem on another forum and it was suggested that I come and visit you guys and see if anyone else is having the same problem. My M&P was purchased in a private sale so I don't have the box or original paperwork with the carbine. I don't know the exact model number but it is a 16" flat-top with a picatinney gas block, dust cover, forward assist, and 6 position stock. Ever since I brought the rifle home I've had feed problems with it. I first thought it was the magazine's fault so I purchased some Tapco, CAA, and Tango Down mags but that didn't solve the problem. I've been using Remington UMC 55 grain FMJ which my RRA rifle seems to adore. The last few trips to the range resulted in my M&P being a high priced bolt action carbine. It would fire the first shot but would have to be manually cycled for the next shot. I went home and cleaned the absolute heck out of it and removed the gas block to see if it was clogged or damaged which it was not. During my next trip to the range I had an idea, why not try higher pressure 5.56 ammo? I bought a box and low and behold it fed, fired, and functioned like it was supposed to. Has anyone else had this sort of issue? Oh yeah my barrel is marked 5.56mm 1:9 and I know that I SHOULD be able to use both 223 and 5.56 ammo. With my carbine this is just not the case.

 

[SSgt M]

Sounds like you have the M&P OR, (Optic Ready)
Have you shot the orig. Ammo since the extreme clean??

I strictly shoot 5.56 NATO ammo with not one problem in the last 1300 rounds using all pmags.
Maybe post some pix, would like to see if it's a Smith gas block. Could also have the wrong buffer, possibly too heavy maybe,
Some of the M&P experts should be along shortly to add.

 

[Breechloader]

If it is used I would look at the firing group springs as well as what ssgt m posted and see if they have been replaced by the previous owner (look at what color they are). Also check that the spring is sitting properly. I have seen posts about one of the trigger spring legs not sitting where it should be.

Sounds like something is "stiff" and not letting the bolt cycle properly.
It might work itself out with a few hundered more rounds or you could contant s&w about it.

 

[cyphertext]

I was thinking the same thing as SSgt M, that maybe someone swapped out the buffer and buffer spring. When you shoot the first round of the .223, does it eject the case?

 

[Remain Silent]

You may have a loose gas key on your bolt and bolt carrier. Remove the bolt carrier from the unloaded weapon and visually check the gas key on top. If the screws are not properly tightened and staked, they may be loose allowing gas to escape and causing the malfunction. You may also have a problem with your gas rings on your bolt.

 

[SSgt M]

Ah....
Gas rings can NOT be in alignment, just like the rings on a piston, I off set mine 1,5,9 o'clock positions.

 

[cyphertext]

If it works with 5.56, wouldn't that indicate that the problem is not with the BCG?

 

[Hobie1]

Welcome!

Hey guys, I had an epiphany. Could you have a partially blocked gas tube? Dirty direct impingement piston? Yes I know you don't have a gas piston system but I'm talking about the part that forces the bcg back.

Glad you headed over this way Bore Rider. These guys are pretty nice and occasionally know what they're talking about.

Regards,

Hobie aka 1hobie

 

[Gunslinger808]

I've got to agree it sounds like a gas flow problem.
Everything that's been mentioned above is a possible.
If you've checked all of the above, then perhaps it's a undersized gas port or crimped (or plugged) gas tube.
Are the carrier key bolts staked in place?
ETA: Hobie1 beat me to it while I was typing.

 

[JaPes]

Everyone beat me to it. If it's firing, but not fully cycling the BCG then everything here makes sense.

Too strong of a buffer spring. Too heavy of a buffer weight. If you have a carbine spring & carbine buffer, try installing them and see how the rifle cycles.

Check the gas rings & gas ring alignment.

Check the gas key.

And if in doubt, call S&W. I wouldn't mention that you're the 2nd owner right off the bat. Take a chance that the first owner never submitted the warranty registration card.

 

[cyphertext]

Take a chance that the first owner never submitted the warranty registration card.

We don't need know stinkin' registration card....

 

[Bore Rider]

The carrier key is tight and the screw heads are staked. The guy that had this gun before me was my ex gf's brother. He was a novice that bought it new, shot maybe a hundred rounds thru it then ran into money problems and had to ditch the weapon. It came with a trijicon tri-power, a hard case, 3 mags, a fake suppressor, and ammo for $1200. Anyway enough history. After the mega cleaning I was able to fire maybe 5 shots with the UMC ammo with no cycling problems. It always ejects the fired round and I had upgraded the extractor with a kit from Brownells too just in case. The buffer spring and buffer are believed to be stock because I purchased a heavy buffer and a extra power buffer spring and the parts in the gun seem much lighter. I originally thought it was BCG rebound and magazine overrun due to too much gas. I even have a carrier weight system that isn't installed and won't be.

When I removed the gas block to check the tube I found no kinks, dents, or blockages. The block itself was pinned to the barrel which leads me to believe it is stock. When I return home to Texas soon I will try some Fiocci (sp?) and PMC 223 ammo to see if there is a difference. I also have a full reloading setup and 8 lbs of Varget waiting on me too. I'm not too concerned about the gun anymore now that I know it works with 5.56 ammo but will continue to check back on here periodically to see what the gurus of the M&P have got in store for me. I will see about posting some pics soon too. Thank you

 

[Hobie1]

Please do return

And tell us you find. We all benefit from getting people's experiences and learn from them.

Have a good one!

Hobie

 

[Gunslinger808]

Inquiring minds want to know!

 

[thndrchiken]

I suspect your gun may be finicky on the ammo, I've heard of a lot of quality issues with the bulk pack UMC Remington ammo. Both in handgun and 223. While I don't have an M&P, my parts build AR chews through just about any ammo I feed it. Haven't tried the UMC though.

 

[danco]

Ah....
Gas rings can NOT be in alignment, just like the rings on a piston, I off set mine 1,5,9 o'clock positions.

This is a myth. The gas rings are compressed when the bolt is in the carrier and the gap in the rings nearly closes. It does not matter if all three gaps are aligned or staggered...

I'd suspect to heavy of a buffer and/or action spring.

A blocked gas tube could also be a possibility.

Functioning with under-powered .223 (vice 5.56) is precisely why the M&P-15 is designed to be "overgassed", so these problems should not be occurring...

 

[jsimmons]

The OP didn't say he modified anything, so the action spring/buffer weights are probably not the issue. I've seen reports of Remington .223 ammo being junk. I would try a different brand (Federal, PMC, etc).

 

[MPDC]

Buffer too heavy is logical. The owner probably set it up for 5.56 and relatively slow cycling. Or, a .22 LR "kit" has clogged the gas system partially. That is why dedicated .22 uppers make more sense than conversion kits.

Buffer would be my first guess.

 

[oneyeopn]

Buffer/Spring or partially plugged gas block/tube. After shooting the Russian Ammo in mine, I couldnt believe just how dirty my Sport became.
I have a H2 buffer in mine and it runs 5.56 beautiful but .223 just dribbles out the ejection port. Glad I dont have anymore .223.

 

[SSgt M]

This is a myth. The gas rings are compressed when the bolt is in the carrier and the gap in the rings nearly closes. It does not matter if all three gaps are aligned or staggered,


How is this a myth, it is taught at the lowest level of basic assembly.
Just asking where you heard this?
....after a little research, it's up for debate on every forum.
Guess I'll do it as a good practice