AR10 failure to lock back and ammo misfeed

[DEB]

Good morning to all. Question I have an S&W M&P10 that I had a new extendable butt stock installed. I took it to the range the other day and it does not feed properly nor does the bolt lock back either at the last round or even manually through the use of the charging handle. I took it apart and my buffer spring is longer than my friend's buffer spring, about 3" matter of fact. I believe that this is the problem, just seeing if others have had this problem and your thoughts? Thanks.

[Pawpaw]

We need a better description than "not feeding properly". What specifically is it doing?

[Bitter Clinger]

Good morning to all. Question I have an S&W M&P10 that I had a new extendable butt stock installed. I took it to the range the other day and it does not feed properly nor does the bolt lock back either at the last round or even manually through the use of the charging handle. I took it apart and my buffer spring is longer than my friend's buffer spring, about 3" matter of fact. I believe that this is the problem, just seeing if others have had this problem and your thoughts? Thanks.

Unlikely the cause IMHO. Is this a new rifle or did you run it before the butt stock change without problem? Did you also change anything else internal to the upper or lower?

[Dadtodabone]

Undergassed? Short stroke, no feed or lock.

[PBR]

does it feed / eject properly just using charging handle? -- have you tried pushing the spring using just the bcg to see how far it goes or if bottoms out too soon -- the gas key should be close to the lower when it bottoms out, but not hit it -- also did you just replace the buttstock only or entire kit (spring/tube)? -- wouldnt be just the buttstock causing that but if not correct spring could be not letting the bcg go back far enough

[DEB]

Thanks for the replies. I purchased a new gun stock where I purchased the firearm and had them replace the parts as part of the sale. 1st time firing the weapon after it was put away for quite a spell, so I can't go back to the store to seek relief. I can load and lock the weapon, but I cannot lock the bolt to the rear. It just will not go back far enough with the charging handle. It will often, but not always, not feed properly, riding over the round but with enough force to partially feed the round with the bolt on the lower part of the round, even putting quite a dent in the cartridge case. It was doing fine for the first 10 rounds or so. I thought at first it was due to the rounds that I purchased, these being soft points. I attempted to use full metal jackets, but same result. Appreciate the assistance, I will purchase another buffer spring and see how it goes. Although this time I will use dummy rounds for safety. Just was looking to see what else you all thought the issue could be, so that I can work up from the spring, in case that wasn't it.

Thanks again,
Dan

[SigM4]

Did the rifle originally come with a fixed buttstock? If not was the entire buffer tube replaced or just the stock itself. When you say you had a new stock put on assume you mean a six position collapsible buffer and stock? You say your buffer spring is longer, but does it have the same number of coils as your buddy's? What about the buffer itself, is it the shorter carbine buffer?

What I'm getting at here is that I'm wondering if you had a rifle length buffer tube before and it was replaced with a carbine length tube. If that's the case you need a new shorter spring and possible buffer depending on what came on it. The AR-10 is somewhat tougher to diagnose than the typical AR-15 as there are variations depending on manufacturer; there is not one universal spec.

[DEB]

Thanks again all. I believe when I had the butt stock replaced they just placed the original spring and buffer back in. I replaced the spring and now it is shooting like a champ. I was concerned it might have been more, hence the reason I wrote in to get other's thoughts.

Dan