So I finally got done with my build and took it out the other day to test out the new gun and had some problems.

First I must say I made some modifications that could be contributing:
-Converted to a Low Profile gas block to allow the installation of a Samson 15" rail
-Changed receiver extension tube to rifle length to allow PRS stock
-Used a home made Buffer Spring Spacer to allow the use of stock buffer and spring. (spacer was made by copying the length and diameter of a S&W factory spacer)

The problem is the rifle would not pick up the next round. It would fire, eject the round, but not pick up and load the next. I came to the conclusion that the rifle was short stroking and the BCG was not traveling far enough rearward to pick up the next round.
I decided to take out the spacer and then the gun functioned with out problems but I could feel the buffer hitting the back of the tube.

Anyone else have a problem when they changed to a rifle length extension tube?

I'm thinking maybe the gas block is not lined up exactly and it is being under gassed and not able to cycle due to a tighter buffer spring with the spacer installed.

My next plan of attack is to try different length buffer spring spacers to try and adjust the tension on the spring. Any thoughts?

Check your port size when you take the block off. I just went through the same issue (short stroking on a carbine length 300blk). If your port is on the small side, open up one size. Do not attempt this until you are sure you are under gassed, and the port is smaller than nominal. Good luck.

STOP

Do NOT shoot the rifle without the spacer. Using a carbine buffer in a rifle RE WILL break something. What's hitting inside isn't the buffer it's the gas key. Check to see if there's any damage.

Reinstall the spacer and run the action with an empty mag in place and see if the bolt still does not lock back. It's very possible that the spacer is a bit too long and preventing the carrier from reaching it's full travel

So did it ever run? Or is this a new gun? What ammo? They do not like freign stuff!

I had a similar issue when I put a syrac adjustable low profile gas block on mine and a prs stock with the s&w parts to do so. I also added a jp muzzlebrake at the same time. The gun was short-stroking even with the gas block adjustment wide open (out of the box new it shot fine). I'm fairly confident the block was lined up correctly. So I drilled my gas port from .065 to .085 and that fixed my problem. I wouldn't necessarily go that big without an adjustable gas block, but I did so knowing I could turn it down if needed. I'd estimate the block is now adjusted to roughly 1/2 to 3/4 open and the gun runs fine.

Thanks for the advice guys, I looked at the gun and running it without the spacer didn't do any damage but i can see that the carrier is hitting the front of the buffer tube threading on the lower receiver. So the spacer is going back in before I shoot it again.

If I manually cycle the gun with the spacer, the carrier group has plenty of travel so the spacer is not too long.

My gunsmith used red lock tight on the muzzle brake so tearing the gun back apart might not be an easy option. I'm not sure if the rail will slide over the brake. I have heard that opening the gas ports up on the barrel is the solution but I may need to try something else first.

I have a few buffer springs laying around, would cutting off a coil help? I feel by shortening the spring half a coil at a time would allow the gas to push the carrier all the way back.

Do not shorten the springs. Springs have to be of the right length and of the right spring rate.

What ammo are you using? Try full pressure commercial ammo and see how that works. Also, check to see if your gas block is leaking. Some will leak a little bit until the carbon buils up seals them between the barrel & block but if it's excessive the rifle will short stroke.

Does the spacer fit inside the spring coils? Are you using the factory spring? If so, the spring is effectively too short and may not have enough oomph to cycle the action. Does the bolt catch the cartridge rim, or is it over riding the cartridge?

I tried using 3 different types of federal factory .308 ammo.
The spacer I made out of a hard plastic solid rod. I work at an FFL Dealer and copied the dimensions from a stock spacer from a different gun.

The spring and buffer are stock and are the same as what comes in the rifle stock

When firing the gun the round goes off, ejects the spent casing, but does not pick up the next round. The bolt is not being pushed back far enough to catch the next round. without having a high-speed camera its hard to say how much more the bolt has to come back.

Next thing to do is check for gas leaks. Are you using the factory carbine weight buffer? Is it possible the new gas block isn't installed correctly and is blocking the gas port?

I'm at work now and just pulled the buffer springs out of three guns. All are brand new m&p10s and all have different length springs. Between the longest and shortest there is about a difference of 2+ inches.

I'm interested in learning more about this issue as I had a similar issue. As I was firing, the rifle would cycle and I'd pull the trigger and nothing. The chamber was empty. But it did not do this all that often, only 3 times out of 100 rounds. Essentially the bolt retracted but re-engaged without picking up a new round.

The only change I made to the rear portion of the rifle was the buffer tube and stock, I retained the stock buffer and spring after a conversation with S&W. I installed the 7 position Vltor buffer tube. The internal dimensions are the same yet it offers one more position and externally it is slightly longer.

I also added the Vltor low profile clamp on gas block. I haven't had the chance to shoot it again as I moved to Texas and I'm still getting settled and learning where I can shoot.

Keep us posted on what you find. Really curious to know about gas port size and if having it enlarged will cure these issues.

Mac

Why in the world did your "gunsmith" use red (or any) locktite? That's what crush washers are for.

__________________
Buy cheap, pay twice.

I honestly don't know, since it won't do much once the barrel heats up.


So like I said the springs in the guns aren't all the same. I swapped out to the shortest spring i could find in a m&p10 and tried that one. It is about 12" long and about 2.5 coils shorter than what came in the gun. I took the gun out and tried it and it worked flawlessly. I don't know if S&W got different springs from different suppliers or what the deal is.

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You may be on to something? But they are not saying! It seems!

Spring pressure is an issue, but without being able to measure the resistance within their working travel, length means nothing in itself as wire diameter and tension come into play. You can stretch one, but it does not give it more power!

All I can say is mine is running good so far with a small gas port enlargement from .068 to .073.

Could this be the same offset effect as a lighter spring in the balance of things? Possibly so?

At work you said you took apart 3 new for sale M&P 10s to find out why yours may be malfunctioning since you modified it?
Hmm.