Jump to content
308AR.com Community
  • Visit Aero Precision
  • Visit Brownells
  • Visit EuroOptic
  • Visit Site
  • Visit Beachin Tactical
  • Visit Rainier Arms
  • Visit Ballistic Advantage
  • Visit Palmetto State Armory
  • Visit Cabelas
  • Visit Sportsmans Guide

LR-308 build: intermittent issues with cycling


Recommended Posts

I've read though this forum for several days before creating an account and have learned of others failures with their builds, now I'm hoping to gain some insight and help with troubleshooting my build.

Full disclaimer, I built this rifle before the plandemic hit by a few months, and I haven't messed with it much since. So some details will be lacking for a minute til I can fux with it some more.

Rifle specs: 80% lower from 80-lower.com, I used their elite builder jig in a mill and mill vise to finish it.

Aero m5 upper, faxon 18" big gunner barrel profile with rifle length gas, slr adjustable gas block, rifle buffer tube, rifle buffer and spring, I believe the BCG is GZP manufactured, details on that are fuzzy, I'll dig it up to be 100% positive on that, but I know they produce them there, standard lower parts kit for LR-308, I forget what hand guard I got, but I believe it's a 15" m-lok guard, either way, it's nice and light, not carbon fiber light though, vortex viper scope 5-25 magnification, pmag 10 round mags.

I purchased the entire builder's kit from ground zero precision and talked with Phil extensively before finalizing my purchase. I've dealt with him before on other builds for ar-15s, so I figured this would be fool proof. Everything went together nicely, and preliminary function tests went fine.

On to the test fire and sighting day, I put one round in the mag an fired the round down range, bolt locked back and I was happy, had the gas block adjusted to the center position like directions stated. I figured I was good to go, so I loaded up a mag and proceeded to sight in rifle. I had a myriad of issues from there on, FTE, FTF, some stove piping even, and it wasnt like it went like that one after another, it's been intermittent. I ran the gun oiled up really well too, sloppy dripping wet like I've read on forums for these builds. I opened the gas up all the way and was still have issues with cycling. This lead me on to trying to troubleshoot and I started reading.

I found that the barrel I received from Faxon was built more to the AR-10 spec, so the gas tube I had was too short and ended roughly 1/8" into the cam cutout in the upper, so I  picked up the longer spec'd ar-10 gas tube for a rifle length gas system, now the gas tube extends about 3/4" into the cutout. That seemed to help with cycling, but I still have FTF every now and then. Ejection issues are gone from what I remember, its been a hot minute since I've picked the rifle up. I remember that once the rifle gets a few rounds through it and starts to heat up is when things start malfunctioning.

More reading got me thinking that the gas port wasn't opened up enough, so I pulled out some indexed drill bits and measured the port along with digital calipers, seemed to be good, I dont remember exact number right now, but I'll be measuring again here soon.

Gas block alignment was spot on, used bore scope to verify, and even had the carbon ring centered on gas port when I changed gas tube.

Next thought was perhaps buffer spring was incorrect, more reading on forums from others issues lead me to believe that I should clip some coils, trust me, that was the last thing I wanted to do, but given that everything seemed to be in spec for the build, I figured why not, maybe it wasn't allowing bcg and buffer to travel all the way back. Going out and firing and still having issues after making the other changes and checking specs forced me to take the measure. I figured if it didn't work, then I'd buy a spring that I know is meant to work for this build.

So here I am, 2 years later, hard to believe it's been that long since I've built the thing, but here are some measurements for younz.

BCG weighs  17.8 oz, my scale only reads to the tenth not hundredth...

Buffer weighs 5.3 oz, rifle length buffer, hard to mess up the length

Buffer tube length measured at 9-5/8", but I did it without using ideal tools for that, a cleaning rod marked with finger tip and tape, ill get the exact measurements later on.

Buffer spring has been clipped down to 10-7/8", I dont remember what the stock measurement was, but let's assume it was wrong from the get go. The timing seemed to be off given that the rim of the brass looked like it was getting ripped apart by the extractor. I'm going to get a new spring, just a matter of knowing where to get it and if I should just get the Sprinco red spring or not.

Don't have the measurement on gas tube, but the new one I bought extends far into the cam cutout. Way better than how short the first one was.

Gas port diameter is currently unknown, I'll get that measurement sometime in the upcoming week, or later tonight given time. I did measure it in the beginning and it seemed to be large enough at that time, I'm doubting that now, again I'll report the info when I measure.

Sorry for the long winded essay, but I promise yall won't have to water board me for info. I want to follow through with this thread until this issue is fixed. It's been at the back of my mind forever, bunch of other poop came up and now I got time to get on with other projects finally.


Link to comment
Share on other sites

36 minutes ago, Armed Eye Doc said:

A couple of things.

How many total rounds through the rifle?

I would replace the buffer spring with the 308AR rifle spring on this page.


There may be more questions later.

I may have 100 rounds or more through it. Got fed up and moved on to more important stuff. I'll order myself the spring from there and in the mean time I'll report back with the missing specs on the rest of the equipment for the rifle.

Link to comment
Share on other sites

Got some measure measurements tonight. Gas port measures .089" with indexed drill bit, calipers actually read .088". I have ~5-3/8" from port to vents on barrel compensator. You experts on here have much better knowledge on gas port size than I do, so is that large enough? Stupid me though, I looked again at the carbon ring around port, turns out I needed to space the gas block .025" forward from the back of the gas block journal. I have that setup properly now and you can see in the photos where the gas tube lies in the cam cutout.

Buffer measures 5.28", dont have any issues with racking back bcg with charge handle and getting it to lock back, but there's the info nonetheless.

Depth of the buffer tube measures 9.683", slightly more than 9-5/8", the photo shows 9.707, but hard to get a good measurement with one hand.

I'm going to order the proper spring for this, but as far as gas port, with adjustments made to gas block, do you think I will not need to enlarge this at the moment until I can test fire again? None of the gas block was blocked from the port, the forward edge of the block was just aligned with the forward edge of the port.... 







Link to comment
Share on other sites

3 hours ago, MB88 said:

Got some measure measurements tonight. Gas port measures .089" with indexed drill bit, calipers actually read .088".

Gas port is too small.  18" rifle gas needs 0.093"~0.096" to run right (for a 0.750" gas block journal size).  Get that Armalite EA1095 recoil spring in there and punch the gas port up, and your gun will run like a champ.  All other measurements look good.  :thumbup:

2 hours ago, shooterrex said:

Barrel diameter @ the gas block???

Faxon runs a 0.750" journal for that barrel.

Link to comment
Share on other sites

Got the new spring in from heavy buffers. There is a visible difference in space between coils, the ar15 spring has tighter coils, where as the lr308 spring is more spaced out and of proper length.

Debated between throwing barrel in my mill, which currently doesn't sit at my residence, or just hand drilling the port out nice n easy like. I ended up hand drilling it. Opened it up to .096" since I have the adjustable gas block in it. The next time I have a chance to get the rifle out to do some test firing and sighting, I'll let yall know how it goes. I'm sure I'll be fine now, thank you for the help again.

Link to comment
Share on other sites

  • 2 weeks later...
On 3/21/2022 at 9:40 PM, 98Z5V said:

When you get it back out there, make sure you're doing the initial tests with that gas block in the wide-open setting.

Roger that.

Just helped build another LR 308.... I dont think this one will have the issues I've had, another rifle length gas system and 18" barrel. This time the upper came pre-assembled and the gas tube landed directly in the middle of the cam cutout.

With that said, finding the time to get out and finally fire my rifle is the most difficult part, a bunch of stuff continues to pop up and delay some range time. When I do get out there, I will update this thread with what's going on. The good thing is now there are two rifles we need to test out and sight in, so the desire is much stronger now.

Link to comment
Share on other sites

23 hours ago, MB88 said:

Just helped build another LR 308.... I dont think this one will have the issues I've had, another rifle length gas system and 18" barrel. This time the upper came pre-assembled and the gas tube landed directly in the middle of the cam cutout.


Happen to check the gas port diameter on that one?  I know it sucks, when they're already assembled, but you'll know going out there if it'll run, if you already know the gas port diameter.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...