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Posted: 3/31/2013 3:57:05 PM EDT
| My buddy just built a new ar15 with a 16” barrel carbine length gas tube. He assembled the lower himself, and I did the upper. At first it would only fire occasionally and rarely cycled. I looked at the lower and he had the hammer spring backwards,so the primer was barely dented. Simple fix, but the cycling remains an issue. I swapped the gas tube and block. No change. Then the BCG and handle. No change. I swapped the spring, buffer, and tube from mine. It seemed slightly better, but still not right. It’s rarely ejecting, reloading or cocking the hammer. Everything seems to load freely but I’m stumped. I’ve tried deferent ammo and mags. |
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Sounds like the gas block is not positioned correctly. First question, is the forend a free floating forend? Is the gas block a set screw or clamp on? If yes to any of these, were measurements taken to make sure the gas port in the gas block is centered over the gas port in the barrel? All too many times when people put on gas blocks they jam the gas block up against the shoulder on the barrel and call it good (I know of one guy who is a certified armorer who made that mistake and then was quite distressed when his $2K build was a single shot). If you measure you may find that the gas block needs to be spaced off the shoulder. Most do. Of all of mine only one or two have the gas block slam up against the shoulder and a couple of them are spaced off the shoulder by as much as .050 inch, most are in the .030 range. If you just slammed on the gas block as far as it would go then that is more than likely your problem. Measure from the rear face of the gas block to the center of the gas port. Measure from the shoulder on the barrel to the center of the gas port. Subtract. Then space the gas block off the shoulder with feeler gauges. Eyeball it top dead center and clamp or set screw it down.
If the barrel was dimpled, remove the rear set screw or clamp screw on the gas block and make sure the gas port drill hole in the bottom of the gas block is centered over the dimple. Tighten the forward set screw or clamp screw, then the rear. Make sure you clean the screw threads and the gas block threads with acetone or alcohol and then use a drop of BLUE Lock-Tite on the screws. Do the front screw first to make sure the center of the rear screw stays centered over the dimple, then do the rear one. If that does not do it, check the gas key and make sure it is not loose and leaking. I assume you have checked the bolt to make sure the gas rings are there and OK. It almost has to be one of those two items, gas block positioned incorrectly or the gas key leaking. The only other things it can be is: 1. the gas tube is installed up side down then it would not work at all, being a total single shot, or 2. the gas tube is too short, or 3. the gas port in the barrel is not drilled all the way through or is otherwise restricted. or 4. the gas port is too small. Check the gas port size against the standard for your barrel length (16) and gas system (CAR and .750). For a 16 inch CAR gassed .750 gas block the gas port should be between .070 and .086 inch. |
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