New M&P-10 Bolt will not lock back shop installed a NY Safe act complient rear stock and buffer tube from an M&P-15. They said it will probably jam but wanted me to take it to the range and fire it anyway it is brand new and I don't feel safe firing it not to mention paying 1,800 bucks for a rifle that the gun shop I bought it from says will jam??? Am I missing something or do they just want me to fire it so they can say I screwed something up and wash thier hands of it...

If what I think I understand is correct, you can use a full fixed rifle stock and tube with a 1.95 in. spacer(which s&w sells), the stock carbine 3.25 in. buffer and spring. You cannot use rifle buffer in this setup.

You cannot use a stock adjustable ar-15 tube as it is too short, pinned or not, as the MP 10(.308) has a longer throw and needs another 3/4 in. or so to cycle even with the carbine buffer.

Take it back!

Don't shoot it. If the stock tube is too short, all sorts of fun things can happen when you pull the trigger.

I'd go back and tell then to re-install the original stock and do whatever to make it compliant on their dime. A piss poor job of them as a gun shop to think it would be okay to mix 5.56 rifle operating parts on a 7.62. If they give you any static about it I'd contact S&W and let them know what's going on and see if they will get on your side dealing with the shop. I'd also file a complaint with the BBB if they refuse to sort it out.

Edit: Another thing, if they give you even the slightest amount of s**t about it(to include rolling their eyes) I'd tell them I'm contacting the Governor and letting them know that said gun shop is possibly endangering the lives of the citizenry by Frankensteining gun parts together.

I was able to chamber a round and fire one shot round two went "Click" with No "BOOM" round was buried in the chamber with some finness I was able to clear the weapon. The gun shop is trying to work with me and solve this problen now they have contacted S&W awaiting a reply. The NY safe act complient rear stock "google" it looks like an "S" shape with a seperate housing that holds the Buffer Assy. as stated before they used an AR-15 Buffer Assy housing which I believe is too short but not by much. Any suggestions???

Sounds like when it comes to ar type weapons; your LGS don't know come here from sic 'em! If this isn't already worked out, I'd get my money back from this guy, and go to someone who knew his business.

The S&W M&P-10 is designed to operate with a carbine length buffer and spring. As the carrier is longer, it will require a longer RE to accommodate this combination. If you are going to run a carbine RE, you need to shorten the buffer. There are a couple of threads in this forum discussing what needs to be done to run a carbine RE and who makes the needed short buffer

OK, I know I am resurrecting a long-dead thread, but I have a very similar problem so thought I'd start here.

First off, my 'new' M&P10 was originally CA compliant, so I've replaced the bullet button with a standard mag release since it is not needed in my state. That's resolved now.

But it also came with some mods done by a previous owner, who says he never shot it. As I was starting to clean it for the first range trip, I found that the bolt carrier group will not lock back when I press the bolt catch levers.

My guess - looking at this thread - is that the Magpul UBR stock installed by the previous owner is too short and/or doesn't have the right buffer and/or spring. Thoughts? Since this was a private purchase, there is no taking it back, so I will need to fix what I have. Should I call MagPul and see what they recommend for the combination of this stock and the M&P10?

Or may I am jumping to the wrong conclusion, thinking it is the UBR stock?

Thanks in advance for your help!

If he changed the tube or the buffer he may have it all jacked up. The 10 requires a longer Carbine length tube than a AR by 5/8 inch or so because of longer travel but uses the stock carbine short buffer.
The 10 comes with a mil spec diameter tube, and if he put a commercial diameter stock on it the tube was changed for sure.

*If* you know what you're doing you can and quite successfully. I built a Maten last year with a Vltor A5 tube, H3 Buffer, and a carbine spring. Also have one with a JP 308 silent captured buffer spring and a spacer I cut down.

The problem is with 308 ARs is there is NO standardized spec. Manufacturer X will differ from Y. Example Armalite AR-10 carbine buffers are the same length as AR-15 carbine buffers (3.25") and need the 3/4" longer AR-10 carbine tubes. DPMS 308s have some with super short buffers 2.5". They can use the standard AR-15 tube as the extra 3/4" of travel is built into the buffer.



When doing the mix master route you can't pick a buffer that's too short...

Thanks, Copemech and BlueOvalBandit. I am working on tracking down some buffer/spring combination to make this work. Have an email in to JP now.

And that photo - wow - that's pretty enlightening!

Honestly, I'd just as soon ditch the UBR entirely if I could but I didn't get the original spring and probably not the original buffer either. And since S&W doesn't sell M&P parts, well... I don't even know how to revert!

Any suggestions for a whole new stock setup that will work correctly out of the box? Stock, spring, buffer - all of which are available now and WILL work with the M&P10?

Thanks again.

Ok so there was a little confusion when I first read this tread since it was building off another poster.

So what I'm understanding is if you manually press the bolt catch to try to lock the bolt back it wont work? Can you look inside the ejection port and visually see if you have enough rearward travel to for the catch to work. If you do you might have a problem with the bolt catch itself. If not we can move to the buffer/spring/tube combo, it can also tell you how much more travel you are off by. If it were really miss matched there is a 3/4" difference in AR-15 travel vs 308 ARs it should be readily apparent. The stock is pretty irrelevant and has not bearing on overall travel of the bolt carrier. Just the tube, spring, and buffer.

And next time feel free to start a new post

Thank you, BlueOvalBandit. Yes, you understood correctly, and I worked through your suggestions, which coincided with delivery of a carbine buffer and spring. I saw that - as you suspected - the bolt was not getting enough backward travel for the catch to engage. So I put in the new .308 carbine buffer and spring - both of which were shorter than were currently in the UBR - and all now seems to be working correctly. I'll be heading to the range tomorrow to see if it all stands up under actual use. Wish me luck!

Thanks again (and to copemech too).

Ok, good I think as we have established that much so far. Now that IS a heavy buffer at 5.5oz, and now I just hope you have enough gas pressure to move it.

If it works with full power hunting ammo, but not with Nato grade stuff you are going to need to enlarge the gas port in the barrel to .075 .

What's the disadvantage to 'too short' a buffer?

Wait until that gas key blows out the back and you will find out!

OK, copemech - you've got me a bit worried now. I hit the range, ran about 60 rounds of NATO 7.62 through the '10, and had no issues at all. Running "standard" carbine spring and carbine buffer (the short DPMS one). This is the one:

DPMS Buffer LR-308 Carbine

•Length: 2.485"
•Weight: 0.238 lbs

Is there good reason to think I may experience a major issue downstream? If so, can you explain why to a novice like me?

If you're rifle is really over gassed and under sprung the bolt carrier may be allowed to travel too far back and slam into the receiver before the buffer bumper stops travel. That is really far back by the way.... This can be solved by adding a spacer behind the the buffer to so the buffer will bottom out before the carrier runs out of travel.

With a buffer that's too long your really can't shorten it without getting crazy, remachining etc. Too short, you just make a spacer out of hard delrin or I've even used 2 quarters to take up a little slack.

Ok, it does seem you may have things working, but the simple math is this. Let us use the number 3/4. So the 308 is a longer bullet than a 223 and the bolt has to move further for things to work. The bolt may also be longer, so they use a tube on the M&P-10 that is 3/4 longer than an AR carbine that normally still works with a standard AR carbine buffer. Which is your STOP, BTW!

Put a shorter AR carbine tube on and you then need a shorter buffer so the bolt can still travel the longer distance needed for the 308 to function properly.

Now the only thing I wonder is what spring is in your gun? Logic would dictate that they use a heavier recoil spring in the 308 as well due to increased recoil, but I have no measures on this. You may want to check into what spring goes with that short DPMS buffer you have for 308.

Hope that helps, as I have no idea why they would change the tube to begin with. The only folke that change tubes around here are the ones going to a fixed rifle length for their application, and it is a couple inches longer than the stock carbine stock and wouls require spacer to use the stock AR carbine buffer and spring.

The AR-15 cartridge is 5.56X45mm. The AR-10 cartridge is 7.62X54. So everything has to be larger to handle the larger cartridge, larger powder charge, and larger generated forces. The bullet is about 27% larger and the cartridge over all is about 17% larger. So the buffer tube is a bit longer and so is the bolt carrier group as a result. So, I would guess the buffer spring would have to be heavier as well. However, with the larger mass of the BCG would compensate for that some what.

This thread introduces some items of interest for me. I'll have to take some measurements, as far as I can between my AR-15 and AR-10 to compare.