AR Sponsor
Posted: 3/19/2012 10:44:06 AM EDT
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I have it fixed now, but I figured you guys might find this interesting.
I recently finally completed my first AR-15, which I built piece by piece and assembled myself, save for mounting the gas block and lining up the gas tube. Turns out this stuff isn't so hard. I'm pretty proud of it. It gets more interesting than this, I promise.
If I did my math right, with a PMag and a crapco red dot that will hold me over until I get money for an Aimpoint PRO, that's $786. The trigger is beautiful, and the GM barrel is pretty accurate. Here's the interesting part though. Over the spring break I took this rifle out camping, and it functioned beautifully for about 100 rounds. No jams except when I tried to feed steel case through it, which it wouldn't. It would eject and the hammer would catch, but it wasn't feeding for some reason, so we just used brass case figuring that AR's are just ammo picky. Well, after the fifth or sixth mag, it started short stroking, extracting and ejecting just fine, but not feeding. This concerned me, to say the least. At the time, I blamed the crappy steel mag I was using. Well, I muddled on with my bolt action AR, shooting clays, soda cans, water jugs, and beavers (yes, those furry little bastards). Eventually it stopped extracting altogether, which was very disappointing to me. The charging handle ejected spent shells just fine, which made me believe something was in my gas port, or my gas tube was crimped. Something along those lines. So, the trip ended, and the day I got back into town, I took it back to the guy who put my gas system on to see if we could figure out what was wrong with it. Now remember, the free float tube in this rifle obscures the gas block, so you can't see it. The rifle was disassembled, and the coating on the parts had hardly been worn in, save for my crappy mag rubbing up on my bolt. Well, at my request, the free float was removed, and the problem was staring us directly in the face. My gas block had pushed itself forward. You could see drag marks from where the setscrews were moved, which means they were tightened just fine, but it still managed to force itself forward. The brass showed no sign of overpressure, no ripped case rims, ejector smear, or extractor marks. Everything was normal. What I'm guessing happened is the force of the redirected gas acting like rocket exhaust imparted considerable force onto the gas block, enough to where it dragged tightened setscrews across my barrel coating. So, he realigned it and loctited the ever living  out of it. The rifle works just fine now.
So, if your rifle starts short stroking after functioning fine, then stops extracting, while you can manually extract and eject reliably, don't  your pants and buy a new BCG/gas system. Check your gas block.
I also got a free box of ammo for my troubles. Now, I have to go to class. Also, first post, what's up. |
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yeah won't do that if your barrel is dimpled properly there are ways to do with out the jig |
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Quoted:
yeah won't do that if your barrel is dimpled properly there are ways to do with out the jig this and it wont hit you wallet that much too 
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WOW this is funny that you posted this. A few weeks ago I was thinking about selling my DD lite rail so I took it off and when I did that I found the same thing you found. My gas block had pushed itself forward and the set screws ground into my barrel. So I re locktited it and tightened the hell out of it. I built this upper myself by the way so I have no one to blame but myself .
The weirdest part of all of this is that I had NO problems at all prior to this discovery lol. I finished building the rifle in october and had put about 400 rounds threw it before I found this. So I have no idea if it happened after the first round or after the last round I fired out of it before I took it apart. I will say that I used the red gel stuff, that came with the midwest industries lo pro gas block. It's a different type of locktite with a different name(I just can't think of it). I used this stuff on something else and it didnt hold up, so I am pretty much blaming this stuff on the set screw fail. I used regular blue locktite the second time around and I think I'll be fine now. |
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I don't think you're understanding the failure mode here. Loctite won't fix it because it's not the set screws backing out that's causing this, it's the set screws not having anything but a flat surface to anchor into. Set screws acting as a compression fit simply don't have enough gripping force to hold the gas block in place. It's like pitching a tent without using stakes and hoping your sleeping bag has enough weight to keep it from blowing away.
You need to dimple the barrel where the set screws make contact so they have a hole of sorts to anchor into. |
The way to stop this is to check the bolt carrier in the upper without a bolt.......I use set screw gas blocks on AR45 builds when I use Troy rails cause that is all the will work....if there is any drag on the gas key....it can force the gas block forward as you have described.slide your carrier in with no bolt and feel if it drags on the gas tube......fiddle with it till it don't....
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| loctite should be outlawed. your gun shouldn't need glue. If you put the right parts together properly and torque everything properly it should hold together until you take it apart which is a pita if you used loctite. check your gear often throw the loctite away grease will actually let you get your stuff torqed down better. and dimple your barrel. barrels are much cuter with dimples anyway. |
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Everything was torqued properly with an accurate wrench. Barrel nut threads were greased. Just because you put something together the way it was designed doesn't mean it won't fail. I'm new, not an idiot.
The loctite will work, and if it doesn't, I will dimple the barrel per all of your recommendations. |
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Quoted:
I don't think you're understanding the failure mode here. Loctite won't fix it because it's not the set screws backing out that's causing this, it's the set screws not having anything but a flat surface to anchor into. Set screws acting as a compression fit simply don't have enough gripping force to hold the gas block in place. It's like pitching a tent without using stakes and hoping your sleeping bag has enough weight to keep it from blowing away. You need to dimple the barrel where the set screws make contact so they have a hole of sorts to anchor into. My barrel was dimpled from BCM, and my set screws weren't as tight as when I first installed them, so yea the "fake" locktite that came with the gas block failed. Edit: I'm not new, and I'm not an idiot, and I'm a few builds in and this only happened on this rifle, so I certainly believe the OP to be right. |
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Quoted: Locktite and heat dont mix well and thats why I would use rockset as it will withstand a hot barrel and gas block. This^ Loctite will just ooze out when the barrel gets good and hot, Rocksett can withstand up to 1100 degrees. ETA...it's also solvent resistant. |
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