I recently purchased an M&P10. Put some optics on it, took it out to the range, and immediately ran into a problem. It wouldn't eject the first round I fired. Not only that, but it locked up entirely, so I had to mortar the casing out to load the next one. I put 4 more rounds through it, each one having the same problem.

Went home, cleaned it, lubed it up, and went back last weekend. Same problem. Today, out of curiosity I loaded a magazine, chambered a round, and started cycling it (Without actually firing, so I don't think it's a gas system issue). First 3 ran fine, but after that, it started locking up again. I tried a different magazine, same problem. 3 will run fine, but after that it locks up. I know the ammunition I'm using is good, it runs fine in my deer rifle and my M1A. This is my first AR platform rifle and I'm starting to believe all the trash talk my grandpa gave it after Vietnam.

I'm using Magpul P-MAGs and surplus 7.62x51, the only thing I did to the rifle was attach optics and replace the collapsing stock with a fixed Magpul one.

If fired and unfired rounds are sticking in the chamber, it is not a gas problem. Sounds like an ammo / chamber incompatibility issue. Take a good look at the chamber with a flashlight. No rough spots or burrs? Try a box of factory commercial ammo from a different maker, and be sure it is brass cased. And make sure the chamber is DRY - no oil. Still have issues, a return trip to S&W on their dime may be in order. My M1a can be finicky with some military ammo, while my CETME will feed and shoot anything.

Larry

Mine has run just fine regardless of what I fed it. Either something is messed up with the gas block or gas tube alignment or there is something wrong with the chamber.

If you are handy with rifles, you can check these things out and maybe fix it yourself. If not, I would recommend using the life time warranty S&W provides.

As far as AR platforms in general and the Vietnam experience, the Army hated the rifle and set it up for failure. They used the wrong powder in their ammo contrary to Armalite/Colt's recommendation to start. They also told the troops they didn't need to clean their rifles and provided no cleaning equipment for them to use.

The powder they used burned more dirty and left a lot of residue in the action. This caused malfunctions and jambs. Not cleaning the rifles exacerbated the problem.

The original M-16/AR platforms had some other issues as well that got addressed over time but the above was the main problems, to the detriment of the troops all because the Army was force fed a rifle they didn't want.

Modern M16/M4/AR platform rifles are very robust and reliable as long as they were built correctly.

First off, I would try some commercial ammo, 308 or 7.62 X 51, your rifle should like both, easy peasy,, if you still have an issue, then you may have a tight or rough chamber,,, the magpuls should work fine, but good magazines are a must for any self shucker...

in that event you must return it to Smith and Wesson for repair

What Would Stoner Do - In His Own Words

If cases won't hand chamber and eject, I suspect a headspace problem not allowing the bolt to unlock. I don't think a mag would cause bolt lockup. Make sure there are no burrs on the bolt lugs. Or the receiver lugs either.

What make of surplus ammo are you using and what do the fired cases look like?

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BCCI Life Member #2068

A headspace measurement of an unfired and fired case may tell you something. Factory ammo headspace may expand 5 or more thousands when fired. An unfired/resized case should have 3-5 thousandths less headspace than a fired case for ARs.