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Posted: 4/28/2008 7:09:29 AM EDT
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I've built a few ARs now, but this is the first time I've ran into to this. I traded for the upper complete on this one. I could send it back, but if I can fix it myself I'd rather do that. I'm not going to mention the name of the manufacturer, so please don't ask. I don't believe in name dropping. I've got the windage almost all the way to the left, and it was still grouping to the right. It was windy here yesterday, but I can't believe it was that windy. I was shooting M855 through a 20" bbl on a A1 receiver. I've done some searching here and have found a couple of threads with pictures of before and after. The only problem is that these pictures don't exist. The idea was taking off the bbl, and re installing it. I don't get this. I can do this myself, but with out the pictures I believe I'm missing something. I'd really appreciate some ideas. BTW, The front sight doesn't appear to be canted, and I've see canted sights before. I also posted this in the troubleshooting section, but it doesn't get near the same traffic as we do here. Thanks, DBAR |
That was going to be my first answer, but.......... Maybe the front of the upper is not square. Take a string/fishing line and run it from FSP to C/L or rear sight@ neutral position and measure any offset at the front of receiver. If it is off as much as you say it should be pretty obvious. Unless someone else has the answer I would say ship it back & let them fix it. |
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Thanks, I'll ck the front sight post with the method you suggested. If it is canted, any suggestions? I can do the work myself vs. a possible 3 week+ turn around time. I like working on ARs. So that's no big deal, and I'll take the risk since the AR is (has been) fairly easy to work on. |
| I got a flat top a few months ago that the top rail does not run true front to back. Never could get it to center up with any rear sight I put on it no matter how I adjusted the barrel and sight base in the upper, If I had rotated it any futher the barrel extension would have been off to much. |
I guess I did misunderstand you. I took the bbl off at lunch and I've never seen a bbl put on with so much torque. No much wiggle room between the upper receiver of the bbl. Very little in fact. I have to believe with the amount of pressure that was on the nut, it had to of been causing some kind of problem. I just hope it didn't affect the receiver in any manor. I never thought that anyone would put something together with that much pressure. I took a lot of force to break the bbl nut free of the receiver. More than I could of ever imagined. |
Damn that sure don't sound like 60ftlbs to me. Well since you have the barrel off you could "mic" the receiver. Do you have a vernier(dial) caliper? A ruler will not do as just 1/100th of an inch could cause it to be "off" I'm off now need to take my medication and lay down. Hope you solve it. Will check back in a few hrs. |
I have calipers, but what part of the receiver are you thinking I should measure? I don't get off work for another 5 hours, but I love working on my ARs. So I'm sure I'll be working on it tonight. |
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Devise some way to measure the amount the threaded section of the upper is in front of of the rest of the receiver. This is actually kinda hard to do. Basically you are looking to see if the left or right side of the threaded portion sticks out farther than the other. I do not know the exact tolerances but to me they should be very minimal. IE: 1-2/1000. Let me know if this makes sense to you or not. Top view of threaded portion upper receiver. _______ _______ = good _______ _________= Bad I cannot draw for shit. LOL If all else fails put it back together and give her a whirl. Might just be the over tightening that was causing it. |
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NP, I am surprised nobody else chimed in with help. I am no expert but it was the only thing that made sense to me. Like I said tough If it is not something simple I would let them fix it under warranty. I know it would suck to be without it for any period of time, but it would be worth it in the long run. 
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FWIW, I had a A2 upper that I had a NIW Colt A2 barrel put onto and they used a receiver vise. When done it had 18 clicks of left windage and this was too much for me. Seems the nut pulled the upper to the left when it was tightened. Does not take much at all to cause the windage issue so you may not see the sight as canted. Took the barrel off and used a barrel vise to re install, it solved the issue and I now have 0 windage. As you said you did a search, I was one that had pix but I have since deleted the pix from the host site. 
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pics please. I'd like to see how you did it vs. how I did, and the results. |
And therein lies the reason you see so many posts saying " my AR needs X clicks of LEFT windage to zero".  Some clown who owns a plastic molding outfit came up with this idea of making a piece of plastic to put around the reciever, grossly overpricing it, and selling the notion that you should clamp the upper and not the barrel when you assemble an upper assy. Total and complete BS.  Clamp the barrel, just like it says in GI Ordnance manuals, and you won't have the problem. Yeah, you might need a couple of clicks of RIGHT windage, (my last build did) but I can live w/that.
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Why does it make a difference whether you use an upper clamp or a barrel clamp? If I were to bolt two pieces of metal together with a bolt and nut, what difference would it make whether I held the bolt still and tightened the nut or held the nut still and tightened the bolt? |
Because when you're clamping the aluminum reciever, and then turning a piece of steel (the nut) bearing against another piece of steel (the collar of the barrwl extension and thus the whole barrel assy) and holding the whole thing in place w/a steel pin in the channel of an aluminum upper; you are asking for that steel pin to dig into the left side of that channel and give you excessive right windage, if you've clamped the upper. It only takes a few thousanths of an inch offset of that pin in the channel to give you a grossly out of spec point of impact. Aa a side note, the more foot pounds of tightening force you need to use to line up the notch, the more likely it is that you're going to get that twist, even w/ the barrel clamped. That's probably why I needed to go to 3 right clicks on the M4 build I mentioned in my first note. As I recall, I had to go to about 50 ft pounds to line up the notch. On the other hand, an A1 upper assy I put togethe awhild back only need about 35 ft pounds, and it shoots dead on at mechanical zero. 
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