LMT MWS 20” 6.5 mm Creedmoor Barrel – Excessive Headspace Concern

[Fire4EffectCA]

I purchased a LMT 20” 6.5 mm Creedmoor barrel a while back and recently fired it for the first time. It functioned fine, but the brass took a beating from the sharp internal barrel extension lugs and the brass deflector. I believe the rifle is overgassed.

I fired 65 rounds of Hornady American Gunner 6.5mm Creedmoor 140gr HPBT. The following are measurements from my RCBS Precision Mic. I measured 10 random loaded rounds and 10 random cases after firing.

Before (Average is -1.9 or -0.0019”) Note: 1.541” is 0 on the RCBS Precision Mic.
-2
-3
-3
-2
0
-3
-1
-2
-3
0

After (Average +6.8 or 0.0068”) NOTE: I have never seen such a wide spread after firing and measuring with a RCBS Precision Mic for 223 and 308.
+7
+6
+9
+7
+8
+5
+7
+7
+7
+5

That’s a +8.7 spread or 0.0087” before and after firing. The brass will not last long with that amount of expansion.

I recently purchased Clymer GO and NO-GO gauges.

Clymer GO Gauge: Marked 1.541”

Clymer NO-GO Gauge: Marked 1.547”

That’s a 0.006” spread.

I measured the Clymer gauges in the RCBS Precision Mic and obtained the following results. Note: 1.541” is marked on the GO gauge and 1.547” on the No-Go gauge.

Clymer GO Gauge: Measured +1 (1.542”)

Clymer NO-GO Gauge: +7 (1.548”)

So the Clymer gauges and RCBS Precision Mic are 0.001” off from each other, but the 0.006” spread is correct.

I currently don’t have a way to remove the dual ejectors so I can’t test the case gauges in my LMT MWS with the 6.5mm CM barrel, but I am concerned I am beyond the No-Go gauge in headspace. I just ordered an AR10 Bolt Assembly Tool from Young Manufacturing and will test headspace when it arrives.

I would be interested in hearing others thoughts about the measurements I have obtained.

 

[RyanScott]

The easy way to know if your LMT MWS barrel is overgassed is to ask yourself if it’s an LMT MWS barrel.

 

[Fire4EffectCA]

The easy way to know if your LMT MWS barrel is overgassed is to ask yourself if it’s an LMT MWS barrel.

LOL . . . I know that's a given.

 

[Fire4EffectCA]

Update

I received a Redding FL resizing die and a Hornady cartridge gauge today and I just tried to resize a few of the fired Hornady 6.5mm Creedmoor cases. I had previously removed the primers and cleaned the cases before attempting full length resizing. I find I have to cam over with my Forester Co-Ax press more than I like to fully resize the cases. The cases are also a very snug fit in the Hornady cartridge gauge and must be pushed in. In addition, the trim to length steps are missing from the other side of the Hornady cartridge gauge

I never had these issues reloading 223 and 308.

 

[hlee]

Factory ammunition is usually on the very small side. Measure the fired cases to the datum line. Set up the die to bump the shoulder back 3 thou or so. Confirm that this allows the resized cases to fit in your chamber. Reload. Don’t worry about the cartridge gauge. Your chamber is your cartridge gauge...

 

[Fire4EffectCA]

Factory ammunition is usually on the very small side. Measure the fired cases to the datum line. Set up the die to bump the shoulder back 3 thou or so. Confirm that this allows the resized cases to fit in your chamber. Reload. Don’t worry about the cartridge gauge. Your chamber is your cartridge gauge...

Thank you for taking the time to reply.

Setting the FL die for 0.003" bump is what I normally do. My main concern is that actual headspace is exceeding the No-Go gauge. I did talk with LMT tech support and they only rely on headspace gauges to determine actual headspace. That is why I ordered the Clymer headspace gauges and a Young Mfg AR10 bolt assembly tool to check per their requirements. I should have the bolt assembly tool by Friday.

 

[Fire4EffectCA]

Update

I received a Redding FL resizing die and a Hornady cartridge gauge today and I just tried to resize a few of the fired Hornady 6.5mm Creedmoor cases. I had previously removed the primers and cleaned the cases before attempting full length resizing. I find I have to cam over with my Forester Co-Ax press more than I like to fully resize the cases. The cases are also a very snug fit in the Hornady cartridge gauge and must be pushed in. In addition, the trim to length steps are missing from the other side of the Hornady cartridge gauge

I never had these issues reloading 223 and 308.

I spoke with a Hornady tech this morning and he checked the drawing for their 6.5 CM cartridge gauge and a trim to length step is not provided.

 

[Constructor]

Your brass is .003-.004 under size. Headspace is ok. It the combination of your normal headspace and brass being undersized that gives you the growth you are seeing.

 

[Fire4EffectCA]

Your brass is .003-.004 under size. Headspace is ok. It the combination of your normal headspace and brass being undersized that gives you the growth you are seeing.

What got my attention was that the RCBS Precision Mic is measuring 1.5487” average and the No-Go gauge measures 1.548”. That is why I want to measure the actual headspace with the No-Go gauge. I had one random case measure 1.549”.

 

[Fire4EffectCA]

I received the bolt assembly tool today and checked headspace with the Clymer gauges. The bolt would not lock on the No-Go gauge, which was welcome news. The bolt assembly tool is very handy and I am glad I purchased one.

Next I will take the rifle back to the range and test out the EZTUNE gas tube. I already tested the three layers of double sided tape on the shell deflector with a 20” 7.62mm x 51mm barrel. It totally eliminated case dents.

Depending one the results I may consider the following.

• Polishing the sharp points on the inner lugs on the barrel extension on the ejector side.
• Removing one coil from of the duel ejector springs.

 

[Fire4EffectCA]

I finally had an opportunity to go to the rifle range yesterday and the EZTUNE gas tube performed flawlessly.

Previously with the overgassed stock LMT gas tube installed I measured the following with the RCBS Precision Mic on the fired ejected cases. Average +6.8 or 0.0068”.

With the EZTUNE gas tube installed I measured the following with the RCBS Precision Mic on the fired ejected cases. Average +4.5 or 0.0045”.

That’s a significant reduction in case expansion.

Amazing what the overgassed LMT gas system was doing to the ejected brass. The addition of multiple layers of heavy double sided tape eliminated the case dents from the brass hitting the case deflector. I still have the heavy cuts in the case necks from the inside lugs on the barrel extension. I need to find a good article on deburring these sharp points. I think cutting a single coil from each ejector spring would also help.

 

[GBMaryland]

I noticed the same thing today with my 6.5 LMT. Beats the crap out of the next of the brass, and also being knew it wasn’t ejecting too well either.

i’ve got a couple of LMT EZTune tubes on the way.

 

[Huskydriver]

I noticed the same thing today with my 6.5 LMT. Beats the crap out of the next of the brass, and also being knew it wasn’t ejecting too well either.

i’ve got a couple of LMT EZTune tubes on the way.

Keep us posted once you get them and try them out... Been thinking of picking one up myself