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First .308 build need a little help with gas/ cycling


phantom01231

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I soak my BCG's in Mobile1 FULL SYNTHETIC for a day or so. Then I wipe it down real good with a rag. When it's time to go bang I either squirt it good with Break-Free CLP or I spay some old Outers Tri-lube in there.

With new rifles, the upper and BCG like to be really wet. The contacting metals have to build a physical relationship before they don't need lube anymore.

Kinda like when edgecrusher met 98Z5V for the first.

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I had some problems like that with mine too. The problem with mine was that the bolt was so tight in the carrier that even soaked with oil it was hard to cycle. After a couple hundred rounds it loosened up, but before that I couldn't figure out why the bolt wasn't locking back and I was having an FTE problem. Once the BCG cycled correctly I worked out a solution to the ejection problem, now everything works like it should.

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Found the issue. Not a problem with the ammo. First I tried some 168 gr and same thing happrned. What I found was apperntly when the barrel was turned down they did not go back and clean up the gas port so there was some flash blocking the hole. Got that cleaned up and there was a dramatic improvement. Still not 100%. Next I found that the adjustment screw on the block was getting blow by. I locked it down with tread lock and she worked just fine. But if I adjust the screw gas starts to leak and problem returns. Any ideas on what to use to seal it up and still allow for it to be adjusted. I was thinking high temp RTV. But other then that she shot like.a dream.

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Is your GB made of aluminum? If so is it possible that the threads are oversize? I have not personally done this because I have not used any adjustable aluminum gas blocks, but perhaps, if you find that the GB threads are worn, you could install a steel thread insert. If this looks like it might be a viable solution for you, I might suggest that you go with one of the solid ones instead of the Heli coil "spring" type. If you find that the threads in your GB are intact, You might try installing a longer screw, that protrudes from the side of the GB and then you can put a nut with the appropritae thread on it. You then adjust the screw using an allen wrench, and, while holding the screw with the allen wrench You tighten the nut against the side of the GB, works as a lock nut.

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Naw the threads are fine. IMO they are a little to course of a thread to be used to keep that kind of presure back. Used the Permatex already thats how I got it to work. Im thinking Im just going to close up the adjustment hole with a smaller set screw and lock it in. Honestly I didnt notice much of a diffrence from having the GB set IAW the instructions and having it full open.

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  • 2 weeks later...

Took the gas block off yesterday, carbon pattern perfectly centered on the gas port, so that is not it.  Decided to measure the port hole size, .063".  What was interesting was the drill bit did not just slide into the barrel, but hit an obstruction.  Maybe a burr from the drilling?  Pushed it through the obstruction and checked the bore.  Looks fine.

Lubed the heck out of things and will try again, after making sure the gas tube is clear.

After clearing out any burrs in the port, I ran an .067" drill through to make sure the hole was cleaned up.  Bore brush cleared out any burrs I might have seen hanging into the bore.

Went out to shoot it today, and it works fine.  Ended up with the JP adjustable GB at 1.5 turns out.  It actually would lock back fine at 1 turn, so went down to 0.5 and had a failure to lock.  Went back to 1 T and had a failure to eject.  Back to 1.5 and it ran like a top.  Brass lands in a 2' circle,

Since I cannot leave well enough alone, I did the infamous "15 minute trigger job" while things were torn down.  NATO ammo does not like the lightened hammer strike.  Commercial .308 worked fine.  Live and learn...........

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<thumbsup> excellent job, good info to pass along to others.

I started shooting with the GB out 4T.  From peering in at it, this is WFO.  Funny thing is, there is precious little perceived feel to the rifle from WFO to 1T.  Only the cycling changed.  Suppose it is the dinky little 16" barrel and the .308.  Guy next to me was laughing at the dust puffing up from the shock wave hitting the ground, and I was sitting, not prone.  It is a loud little sucker.

Next up is switching over to the Criterion 18.5" barrel.  Any change in character will be interesting.........

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