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Posted: 8/13/2015 6:24:58 PM EDT
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Finally completed my A4 clone and got it to the range today. I had to adjust the windage ~20 clicks to the right in order to get it to zero at 25y. I'm not sure what's considered excessive or out-of-spec, but I've never had an AR be that far off from mechanical zero before. The front sight doesn't seem canted.
ETA: This is using a carry handle sight. |
| The rifle is a factory Colt, only thing I replaced was the furniture. Carry handle came off of a 6920 supposedly, but I've tried two different carry handles with identical results. The rifle groups fine, I just feel like I'm on the ass-end of the windage adjustment. Wondering if this is something worth messing around with, just sending it back to Colt, or whether it's nothing to worry about. |
| I know what I would do. I'd pull the barrel and square the front of the receiver with a lapping tool sold by Brownells or Midway. I've seen lapping the front of the receiver correct your type of problem many times. I had several builds do that but squaring the front of the receiver fixed them all and since I started squaring all my receivers before building now, I have never had that problem again. |
| Sometimes the barrel rotates while torqueing and the indexing pin on the barrel extension eats into the aluminum receiver resulting in a canted sight. That figures in your case because it throws the front sight to the left which requires adjusting to rear sight to the right to compensate. I had one do this and the armorer who built it laid the rifle down on a board a certain way, pulled out the biggest rolled-leather mallet I've ever seen (had to be 5" diameter), gave the front sight a whack and handed me back the rifle. The next day at the range it zeroed on center. Damndest thing I ever saw, but very possibly your problem. |
| Upper may need lapping. If the face of the upper is a few thousandths higher on one side it is multiplied down the length of the barrel. If you had a free-float handguard the barrel would be off center down at the muzzle end. Tool at Brownells is like $35, plus the compound. I do this to every upper I build to eliminate this as a potential issue before I ever assemble it. Not many need it, but the last one I built did. During the lapping the 9-12 o/clock area of the upper face was untouched while the rest of the face had the anodizing removed. Reminded me why I do it to all of them. |
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