Gunsmithing question on mounting barrel in LR 308 upper

[Luky]

I have a DPMS LR 308 upper and a 24" Lilja heavy barrel, JP brake, along with a PRI CF round forend. I had taken it to a shop to have it assembled "the right way" as I have only done AR15 uppers a couple of times. Alas I think they did a half assed job of it. I broke in the barrel and have been trying to do load development and the best that I can achieve is about 1 MOA with my hand loads (or 168 GMM or 168 BH match). It might put 2 touching, than one up on the left, one an inch to the right and the 5th touching the first two. Can't get any kind of consistency in my groups. Tried different loads, 168 and 175 SMK, two different powders, etc. I can get 5 rounds touching at 100yards with my 24" savage and my 175smk/43.4gr Varget load (also my best groups in the LR 308).

So I decided to reseat the barrel and took it apart. First I found they did not mount the JP large comp (Bennie Cooley type) with the rocksett I had provided but used red locktite instead. This is beside the fact that they did not time it correctly and the flat top of the comp was not parallel with the ground when shooting (about 15 deg off). I removed the Wilson combat gas block and was surprised how deep the marks are from the set screws (seems too tight to me). I set my torque wrench to 30 ft-lbs and it did not even click when removing the barrel nut. I think I wasted $80...

So I am going to set the barrel and notice there is a bit of slop between the barrel extension and the upper. I have read of using Devcon to bed the barrel (I don't have this) and I have also heard of using Loctite as well, but it could be difficult to remove the barrel. How about using Rocksett? Supposed to soften up when soaked in hot water and is not a high torque "glue" like red locktite, but should fill the gaps and resist the heat at the chamber.

Also when torqueing the barrel nut should I use a bit of anti seize on the threads or leave them dry (or blue locktite/rocksett)? The PRI barrel nut instructions are to torque to 40 ft lbs and loosen (three times) then torque to 40 ft lbs (and past as needed) to line up gas tube.

Anything else I should do?

 

[WhiteDingo]

Anti-seize on the threads is a good idea for sure; it will make it much easier to remove later if you need to. Other than that, pre-seating the barrel nut as you described is not a bad idea on a new upper/barrel nut combo, just to ensure that you're getting a good tightening on the barrel extension flange.

The torque applied to the barrel nut does seem a little light. I'm not saying that this was your problem, but it probably wasn't helping the accuracy much.

 

[bptactical]

Clean and dry all of your parts. Install the barrel and snug the barrel nut lightly, maybe 10 ft/lbs or so. Grab the muzzle and see if you can feel any movement relative to the upper. There should not be any, if there is some then yes you can bed it or Loctite it. I would think the Rocksett is too brittle being a ceramic type product. Most people I know that have Loctited a barrel have considered the entire upper as a consumable item.
Make sure the receiver ring is true.
Now when you go to mount the barrel for good install the barrel nut dry and bring the nut to 40 ft/lbs. Do this three times, backing off the nut a few turns each time. This burnishes the threads a bit and the nut will pull up to torque accurately.
Lube the extension threads with a dab of Aeroshell grease or a non graphite anti seize.
Torque it to 40 ft/lbs initially and check the gas tube alignment. Never loosen the barrel nut to align the tube but don't exceed 80 ft/lbs. Some say 55 ft/lbs is a magic number.
Anybody that Loctites a barrel nut does not understand the platform, the barrel nut is locked in place by the gas tube and a properly prepared and torqued nut is not going to go anywhere.
When I install a gas block the bottom flat of the gas block is checked for level against level of the upper rail surfaces. Just because it wasn't level when you prone out doesn't mean it's not level relative to the rest of the rifle.
A touch with a mill to spot the barrel for the setscrews.
I use red Loctite for gas block screws all the time with no issue and don't crank the screws until the wrench is ready to shear. An overtightend fastener is just as bad as under.

 

[Luky]

Thanks for the feedback guys.

I cleaned everything with a spritz of brake cleaner and paper towels (takes the grease/oil right off). I wound up using the rocksett to bed the barrel as I figured it would be easier to remove than Loctite (should I ever need to) and just needs to fill up the gap. There is no real "movement" (other than vibration) once the barrel is torqued down so I think it should be enough. There was definitely more than a slip fit between the upper and the barrel extension. I turned the upper so it was vertical in the vise blocks (figured this would help to keep the barrel centered while "bedding") and wiped a thin layer around the inside of the receiver with my finger and set the barrel in, wiped off the little bit of excess around the threads (also scraped a little out of the inside of the receiver with a small flat tip screw driver when I was all done). Then I mounted the barrel nut and torqued it to 40 ft lbs and loosened it three times per the directions from PRI. On the fourth time I put a dab of anti seize on the threads and went to 40, checked the gas tube, went to 45 checked again. Somewhat of a PITA as the PRI wrench is a lolly pop shape that goes around the whole barrel (has four pins that hold the nut) and had to be completely removed to try the gas tube fit (but it is a very nice/robust steel tool and has 3/8" & 1/2" holes to mount a torque wrench, also has pins on the other side for KAC barrel nut - would have to recommend this tool). Finally got there at 70 ft lbs and the tube slid right in (good thing as my bench had started moving). Rotated the upper back to parallel with the ground (Used a level on the upper and flat top part of gas block) and mounted the gas block with rocksett on the set screws (my barrel already has a flat for the gas block set screws), just snugged them down. Used some 400 grit paper set on our corian counter top to take a bit off the comp (kept rotating the comp as I did the polishing strokes) to get it to time right (again used a level on the upper receiver and a level on the flat top of the comp). Took me 4 tries of polishing but got it timed correctly now and mounted with rocksett... Even this I think is better as when the shop did it one side of the comp with flush with the barrel and one side had a ridge (they told me it must have been drilled off center when I asked). The barrel is .840" and the end of the comp is .850" so I expected a tiny bit. But when I got done with my work, I now have a tiny lip all around the comp where it meets the barrel (enough to catch your finger nail on) which makes me think they must not have machined it perpendicular to the threads (they had to remove about 0.050" for the timing) and I lucked out in that my careful hand polishing job improved it. I remounted the forend and my scope, then back in the safe.

Now I am priming brass so I can load some rounds tomorrow. Maybe I can hit the range next weekend. Looking forward to seeing if it actually makes any improvement in my group consistency. At least I feel better about doing it myself and knowing exactly how it was done... Only thing I wish I had was a tool to lap the face of the upper just to "know" that it was square to the bore as well.

 

[rero360]

To piggyback on this thread if I may, would Tetra Gun grease work for greasing up the barrel extension and barrel nut? Looking at their website, they say it is a Heavy Duty Fluoropolymer Grease. I can't find any literature stating that it has graphite or copper in it. Reason I ask is that's what I have on hand and need to know if I can use it for the barrel install on my LR10 or if I need to use something else. Thanks.

 

[bptactical]

To piggyback on this thread if I may, would Tetra Gun grease work for greasing up the barrel extension and barrel nut? Looking at their website, they say it is a Heavy Duty Fluoropolymer Grease. I can't find any literature stating that it has graphite or copper in it. Reason I ask is that's what I have on hand and need to know if I can use it for the barrel install on my LR10 or if I need to use something else. Thanks.

I wouldn't use the Tetra if you have a choice. Use a moly based grease or anti seize. I use the Tetra all the time for slides and such but I don't feel it has the extreme pressure qualities you want for a threaded connection.
Go to your local auto parts store and get the small packet of anti seize they sell for spark plugs, all of the PermaTex brand ones I have seen here are the mica based stuff and are graphite free.

 

[rero360]

Awesome, thank you bptactical.

 

[smschulz]

At least now you've realized that installing the barrel isn't rocket surgery.

Nothing wrong with redoing the installation but there are always multiple factors to accuracy and it is always a challenge to zero in on the correct one.

Good luck and keep us posted on the results.