AR tuning gurus….need your help.

[pmclaine]

I own an LMT CQB.

It came with the standard 16 inch or so carbine barrel.

I came across a sweet 20 inch stainless steel barrel in the PX and bought it. I have probably fired 1000 rounds through it. Shoots great.

Any way, last night while cleaning the rifle I note a ghost imprint of the back of the gas key on the top of my lower receiver at the buffer tube.

May have been there since I fired my first round on the rifle.

I don’t like it.

Last thing I want to do is beat my lower or shear off the gas key.

Gas block on the twenty inch barrel is slightly forward of the 16 inch barrel gas block by maybe an inch, just enough to have it sit past the end of the quad rail. The sixteen incher is in the quad rail IIRC.

I’m thinking the extra gas going through the tube before the bullet exits the barrel is causing this.

Liking the twenty inch barrel but not liking the impact on the lower receiver.

Perhaps a heavier buffer or spring but What might do is cut the twenty to 18 inches which is what I really wanted for a barrel….hmmmm.

 

[1911hombre]

1 (one) quarter in buffer tube. As in 25 cent piece. AKA two bits. Look at it as insurance. Others will soap-box better on this than me.

 

[Evintos]

Inspect clearance available at full lock back and then look for BCG overtravel past bolt catch lock (example image below). Remove the charging handle, leave BCG in, insert empty magazine, with a pen, punch, or finger (be careful with the finger) move the BCG back until it is locked to the wear with empty mag. There should be a gap between the castle tower and the rear of the gas key. The rear of the gas key should be roughly half way or past midpoint but not touching the castle tower at lock back. If it is close to or almost touching the receiver, you should decrease stroke length.

1. Buffer bumper too soft or too short. Incorrect durometer material - Replace with new bumper or add a washer/spacer
2. Incorrect buffer body length (too short) - Replace with appropriate length buffer or add a washer/spacer
3. Gas key dimensions too long. - Replace with appropriate spec gas key.
4. Excessive gas causing buffer bumper compression. - Increase buffer weight.

 

[pmclaine]

Impact mark….

Buffer looks like it is bottoming…

Length of buffer

Spring length 10.25 inches

 

[XLR308]

The transfer mark is really minimal but that would bug me to.
What does the buffer length measure with calipers as opposed to the steel ruler pictured.
With the round count on it maybe it has lost a small enough fraction to just allow a small amount of contact.

 

[pmclaine]

Bolt at lock back….no sign of gas key.

Still forward of charging handle cut….

 

[pmclaine]

Changed out gas rings that were looking thin on ends and replaced cam pin that was developing a wear line I could catch with a nail…..

Noticed last night bolt would collapse from the weight of the carrier. Rings will fix that.

Bought a new take off LMT BCG last night out of the PX for spares.

 

[XLR308]

You need to have a visual on the carrier at full compression in the extension and the gap that should exist can be measured.
Not sure if the LMT AR10 is the same dimension as the AR15 but there is a go and no go distance.

 

[hkguns]

Take your carrier out of your rifle and run it into your buffer tube with the spring and buffer. Push it down as hard as you can. It will go mostly all the way, then a little further if you bear down. You need to measure it at the bear down point.

If you don't have two US quarters worth of clearance, as stated above, toss a quarter down the tube and then install your spring and buffer. I'm guessing you have far less than 2 US quarters worth of clearance or a very soft / worn bumper on your buffer.

 

[pmclaine]

The transfer mark is really minimal but that would bug me to.
What does the buffer length measure with calipers as opposed to the steel ruler pictured.
With the round count on it maybe it has lost a small enough fraction to just allow a small amount of contact.

Only own 6 inch calipers….

Best I can do for measurement….

 

[XLR308]

Only own 6 inch calipers….

Best I can do for measurement….

View attachment 8126398

View attachment 8126399

Was reffering to just the buffer, but sounds like the polymer bumper is just a bit worn like Evintos mentioned is his list at #1.
The original durometer that the material originally had has softened a bit or just shortened slightly from repeated compression during cycling.

Edit: Most likely all you need is a new polymer buffer pad or the quarter drop in suggested.
If it were me i would want it back to original spec or at minimum a real quarter made of silver not the bullshit we call quarters now.

 

[6.5SH]

Best visual guide I've seen:

 

[pmclaine]

Best visual guide I've seen:

That is a great visual.

Painted my transfer mark with a Sharpie so it’s all fixed now……

And just to hedge my bet bought a new buffer spring, buffer and parts to replace the other spares used.

I’ll replace buffer/spring and see if my sharpie goes away.

Checking records gas rings had about 1700 rounds. Bolt will hold weight of carrier now.

Cam pin had under 5000 rds, probably would last forever but if it lost enough metal to catch a nail why not go “fresh”.

I’ll measure new buffer against old, ditto buffer spring.

I’d consider this a low round count rifle, surprised wear parts get changed that often.

Keep AR parts on hand.

 

[n2ishun]

It's over gassed.
Install adjustable gas block, turn gas down.
Call it a day.

 

[pmclaine]

It's over gassed.
Install adjustable gas block, turn gas down.
Call it a day.

It’s an LMT….over gassed you say?

I like my ejected brass distancing by 8 feet because it might have COVID.

 

[Aftermath]

It’s an LMT….over gassed you say?

I like my ejected brass distancing by 8 feet because it might have COVID.

See...it is definitely over gassed. That COVID distancing is only 6 feet.

 

[bigjake83]

LMT's are notoriously over gassed, just keep in mind you have a few different option based on your capabilities to resolve this.. Adjustable Gas Keys, specialized Gas tubes and as stated above and always the best option Adjustable Gas Block.

 

[ToKeepAndBear]

Black River Tactical 50:50 gas tubes made a big difference in my LMT MRPs.

 

[pmclaine]

Got my parts coming this week.

Found a new BCG in the PX this week.

Thinking the buffer spring and buffer will give me two quarters clearance.

I love the idea of adjustable gas block and other good ideas.

Likely what I will do when I decide to send one of my barrels off to have the extension put on a good barrel.

 

[MontanaMan]

LMT's are notoriously over gassed, just keep in mind you have a few different option based on your capabilities to resolve this.. Adjustable Gas Keys, specialized Gas tubes and as stated above and always the best option Adjustable Gas Block.

AGB's.......................the easy button.

MM

 

[tomme boy]

Be careful which gas block you buy. Some will need to take the hand guard off to adjust

 

[pmclaine]

Let’s close this circle..,

New buffer/spring on top…old is a little short…

Length of buffer old first…

Than new….

Wait….WTF?

Bolt pushed into buffer tube with old spring/buffer.

With new set up….

Got my replacement rings, new cam pin and scored a complete new take off LMT bolt out of the PX for a Hundy.

I’m thinking the worn gas rings are the biggest factor seeing as both buffer set ups seem to have plenty of clearance unless that old buffer bumper is so soft that under recoil it squashes like Diane Fientseins face currently looks.

Still the new buffer being shorter than old is worrisome.

Otherwise….Feels good to “freshen” up the AR some.

 

[Aftermath]

That polymer bumper does wear out. Looking at all this so far, my GUESS is that is the issue.

 

[pmclaine]

That polymer bumper does wear out. Looking at all this so far, my GUESS is that is the issue.

Length doesn't matter though. I'm not saying that because I'm Irish.

My old buffer is longer than new. It also looks like I got more Clarence with the old parts despite the shorter spring.

But I'm thinking the old buffer functions like a hard smashed vagina.

Have to watch it but hope the longer spring and fresh dong tip keeps the gas key from impacting the lower.

I hardly consider this a hard used rifle, check your gear.

Next step will be a heavier buffer if need be.

 

[Ernest 5.56]

You can check the gas key clearance without the spring. I think that's the preferred way.

 

[pmclaine]

You can check the gas key clearance without the spring. I think that's the preferred way.

It would certainly make it easier to take cell phone pics and check the two quarter Clarence.