12" LMT piston issue- bolt wont lock on last round

[Tangodown911]

Switched over from a 10.5" DI setup that ran flawlessly on my CQB to the 12" piston kit. It ran fine for approx. 100 rounds. Cleaned it then experiencing a failure for the bolt to lock back on the last round most of the time...but varies according to lubricationit seems. This only occurrs with suppressor mounted in both supp. and fire modes... Wtf

To fix this I have tried...
Standard buffer
H buffer
H2 buffers
3 different springs
Various loads of ammo
30rnd Pmags
20rnd Pmags
A tapco 30rnd mag

What I have noticed:
When the gun is clean is will almost never lock back on the last round. If I strip off the CLP and frog lube (run separately) the gun then has a 50/50 chance of behaving right and locking back on the last round.

Called LMT and they recommended a H3 buffer so I have one on the way now. You guys have any pointers?

 

[problemchild]

Re: 12" LMT piston issue- bolt wont lock on last round

Where is the brass landing? What position on the clock? Is there any odd extractor marks on brass rim? Where are the brass marks on the upper brass deflector and are there any marks on the ejection port itself?

Sounds like you might need an adjustable gas block so you can tune in the right amount of gas.

Does LMT guarantee a suppressor operation on it?
How does it run without the can on it?

 

[Lockdown1243]

Re: 12" LMT piston issue- bolt wont lock on last round

Posting for Tango, his gun runs fine without a suppressor on it. It's looking more and more like it is WAY over gassed. I'll ask him about the brass or have him post.

Also, the gun cycles fine in suppressed mode without the can on it as well. That's why I'm starting to think it's possibly over gassed.

 

[Tangodown911]

Re: 12" LMT piston issue- bolt wont lock on last round

Brass landing is spotty. Sometimes 3 feet away at 1:30 and then 10-12 away 2:30.

 

[SuckitTrebek]

Re: 12" LMT piston issue- bolt wont lock on last round

sounds like a mag issue. yes you tried tapco and magpul. now try usgi.

 

[S.T.A. Training Group]

Re: 12" LMT piston issue- bolt wont lock on last round

The brass landing at 1:30-2:00 means it's getting a lot of gas but not locking the bolt to the rear could be a bolt catch that is slightly out of spec, the gun is short stroking (which does not appear to be due to brass ejection), or bad mag which I'm sure you've tried multiple.

Put a mag in with one round and have someone watch the BCG cycle when you shoot and they should see if the rifle is short stroking it won't go back all of the way and won't lock to the rear. It is probably just a mil spec bolt catch that is a little off.

 

[Tangodown911]

Re: 12" LMT piston issue- bolt wont lock on last round

I'll try Usgi today. But the reality of it is if the thing won't work with Pmags then it ain't worth owning. I'm leaning towards the piston kit being way over gassed I know the bolt is coming all the way back because when I fire it and hold the bottom of the mag catch it does lock back. So I doubt it's a short stroke issue. But a over gas issue.

 

[S.T.A. Training Group]

Re: 12" LMT piston issue- bolt wont lock on last round

I agree the Pmag is great but not perfect and there have been a few changes over the years. Are the PMAGS new (year or newer) or are they older ones? Try both if you can. Don't over look the bolt catch issue bc I have had several rifles with "Milspec" bolt catches that were off and it was a pretty simple fix. I know this is a frustrating thing to go through. Hope you get it worked out..

 

[Tangodown911]

Re: 12" LMT piston issue- bolt wont lock on last round

Pmags bolt catches and the rest of everything else in this gun worked before the piston kit was installed.

 

[frankythefly]

Re: 12" LMT piston issue- bolt wont lock on last round

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: STA Training Group</div><div class="ubbcode-body">I agree the Pmag is great but not perfect and there have been a few changes over the years. Are the PMAGS new (year or newer) or are they older ones? Try both if you can. Don't over look the bolt catch issue bc I have had several rifles with "Milspec" bolt catches that were off and it was a pretty simple fix. I know this is a frustrating thing to go through. Hope you get it worked out.. </div></div>

I have a 16inch piston LMT and I have issues running suppressed on my older PMAGS. New Pmags bought in the last month work fine, however my olders ones which are 1-3 yrs old are doing the same as mentioned in the thread.

note: my lower is a seekins.
Running H2 buffer, PWS buffertube, etc.

Do you have a full auto carrier or a just the regular one?

also,
Tried USGI mags which are anywhere from 3-20yrs old and they worked fine.

 

[Tangodown911]

Re: 12" LMT piston issue- bolt wont lock on last round

Full auto carrier...and I'm currently armed with various Usgi mags enroute to assault some dirt and test this mother

 

[Tangodown911]

Re: 12" LMT piston issue- bolt wont lock on last round

Still no dice with USGI mags. If gun is clean and lubed then it won't lock back period. Once it got dirty then it was locking back about 50% of the time. It still ejects shells all over the place from 1:00 to 3:00, 3 feet to slinging them about 10 foot away. No method to the madness. Was using federal brown box XM193 today. Used a different spring than previously tried as well as a H3 buffer. I cleaned it and lubed it then it went back to 100% failure.

This thing has got to be way over gassed.

I have a feeling that this thing is going back to LMT like almost everything else I've bought from them.

 

[Lockdown1243]

Re: 12" LMT piston issue- bolt wont lock on last round

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Tango__Down</div><div class="ubbcode-body">Still no dice with USGI mags. If gun is clean and lubed then it won't lock back period. Once it got dirty then it was locking back about 50% of the time. It still ejects shells all over the place from 1:00 to 3:00, 3 feet to slinging them about 10 foot away. No method to the madness. Was using federal brown box XM193 today. Used a different spring than previously tried as well as a H3 buffer. I cleaned it and lubed it then it went back to 100% failure.

This thing has got to be way over gassed.

I have a feeling that this thing is going back to LMT like almost everything else I've bought from them. </div></div>

You have luck with LMT's like I have luck with Ex-Wives.....

 

[Tangodown911]

Re: 12" LMT piston issue- bolt wont lock on last round

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lockdown</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Tango__Down</div><div class="ubbcode-body">Still no dice with USGI mags. If gun is clean and lubed then it won't lock back period. Once it got dirty then it was locking back about 50% of the time. It still ejects shells all over the place from 1:00 to 3:00, 3 feet to slinging them about 10 foot away. No method to the madness. Was using federal brown box XM193 today. Used a different spring than previously tried as well as a H3 buffer. I cleaned it and lubed it then it went back to 100% failure.

This thing has got to be way over gassed.

I have a feeling that this thing is going back to LMT like almost everything else I've bought from them. </div></div>

You have luck with LMT's like I have luck with Ex-Wives..... </div></div>


Lol. Yeah I feel like throwing this thing off of a bridge right now. I'd feel better about this if I did. Not to give you any ideas.

Anyways. It's boxed up and will be headed back to LMT.

 

[frankythefly]

Re: 12" LMT piston issue- bolt wont lock on last round

I have an extra gas regulator for the LMT. I am going to enlarge the port and see if I can fix the over-gas problem that way. I tested some more of my USGI mags and only 1 held the bolt back.
I figure a I will just drill a larger hole until it functions correctly.

Anyhow let me know how it works out with LMT. I believe it is going to be a very hard problem to diagnose or fix without having your can. Each can functions a little differently in relation to back pressure. That and I am sure barrel length, ammo and a dozen other things come into play also.

 

[Alpine 338]

Re: 12" LMT piston issue- bolt wont lock on last round

I had and still have the same issue on my first generation LMT gas piston with 16" barrel. It does the same thing, or doesn't fully eject the case when using cheap low powered commercial ammo. With real MilSpec ammo I have no issues. My 12" and newer 16" gas piston uppers both run without a hitch. Using the H2 buffer with all of them.

 

[Tangodown911]

Re: 12" LMT piston issue- bolt wont lock on last round

I talked to Gene Swanson yesterday about the issue. I said he would work on fixing it. I don't want to bore you guys with my experiences with lmt, but I will say that it's been ridiculous trying to get stuff that works as it should from them.

I am currently battling the bad batch or faulty 18" SS barrel issue right now and am waiting until the end of April before they have any 18"SS barrels. I was told by lmt the cut up their 5.56mm SS barrels because they wouldn't shoot, I guess I got a bad one because it wouldn't shoot any better than 2.5" at 100 no matter what I put in it. But most groups were 3.5"-5" and some were bigger than that. I sent the barrel off Gene found a bur in the rifling near the crown...recrowned it and sent it back with test targets which grouped 1" give or take with black hills MK262 ammo. I shot it with several different loads and ammo. Fail. It really didn't shoot any better. Best group was with 55grn tap that was as little less than 2".

And in case you are wondering...yes I can shoot. I have several ARs that I can get an 1" out of regularly and these are basic carbines like RRA And Bushmaster. And I have more than one bolt gun that is sub half MOA. Anyways enough of the resume. I had a 10.5" lmt barrel than shot sub MOA with 77smks, so I know they can make a quality product when they want too.

I did email Karl Lewis about the SS barrel issue so I know he is aware of it. Maybe someone there can get all of these issues under control. It's sad because until now I have been a die hard LMT fan because of their engineering, quality, and customer service. And kudos to Ryan Johnson at LMT. He has been most helpful and actually called my back and has tried to help out as much as possible with these issues. He's good people in my book.

 

[Tangodown911]

Re: 12" LMT piston issue- bolt wont lock on last round

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: frankythefly</div><div class="ubbcode-body">I have an extra gas regulator for the LMT. I am going to enlarge the port and see if I can fix the over-gas problem that way. I tested some more of my USGI mags and only 1 held the bolt back.
I figure a I will just drill a larger hole until it functions correctly.

</div></div>

Frank are you referring to the selector on the gas block? If so, if you drill it out more wouldn't that let more gas through causing the gun to operate faster. My issue is the opposite I believe. Less gas or smaller ports in my case may be a fix for me. Also when I spoke to Gene he said the piston barrel I got was not tested by him, and he seemed certain it was tested by someone there before it left the factory. These piston kits have been out for a couple of years now, but it's strange they still have issues and can't get it right.

 

[Tangodown911]

Re: 12" LMT piston issue- bolt wont lock on last round

I also talked to Clint at heavybuffers.com and he recommended the XH buffer which is 8.5oz. with a Wolff spring to tame this thing down. All delivered for $150. I'm not ready to drop more money into this thing. One would think it should work fine straight from LMT even with a suppressor. After all, it was designed for a suppressor.

Clint said these issues with the LMT piston are typical and he sees his share of the issues. I wonder if LMT isn't hearing enough feedback from the end user for problems like this. I also said he was aware of the SS barrel FUSTER CLUCK ( FUSTER CLUCK are my words not Clint). Man am I the only guy who didn't know of the SS barrel issue?

 

[frankythefly]

Re: 12" LMT piston issue- bolt wont lock on last round

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Tango__Down</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: frankythefly</div><div class="ubbcode-body">I have an extra gas regulator for the LMT. I am going to enlarge the port and see if I can fix the over-gas problem that way. I tested some more of my USGI mags and only 1 held the bolt back.
I figure a I will just drill a larger hole until it functions correctly.

</div></div>

Frank are you referring to the selector on the gas block? If so, if you drill it out more wouldn't that let more gas through causing the gun to operate faster. My issue is the opposite I believe. Less gas or smaller ports in my case may be a fix for me. Also when I spoke to Gene he said the piston barrel I got was not tested by him, and he seemed certain it was tested by someone there before it left the factory. These piston kits have been out for a couple of years now, but it's strange they still have issues and can't get it right.

</div></div>

Your right, for some reason I thought it would bleed the extra gas off. If you look at the regulator on Supp mode it has the large hole, that why I was thinking it needed to be bigger. Anyhow, the extra regulator I have is the old gen which only has a hole on one side. So maybe if I drilled a small hole on the opposite side if that would work?

 

[Tangodown911]

Re: 12" LMT piston issue- bolt wont lock on last round

UPDATE.

Apparently LMT found an issue with it after test firing it. It failed to lock back a couple of times. They replaced the barrel, test fired it with a suppressor mounted and found no issues. They used an H2 buffer.

For you guys that have issues with them you may want to give them a call.