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Posted: 7/9/2004 8:47:27 AM EDT
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Hey all, When re-barreling an upper how much windage is considered too much when you are done? Consequently, if the windage is excessive, can anything be done to to get things closer to mechanical zero? Thanks |
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Almost always, "too much windage" is required to the left---and almost always, it's because the barrel twisted during tightening the nut, and the steel locator pin on the top of the barrel dug into the left side of the locator channel on the aluminum upper. It only takes a couple of thousanths off being off at that point, to move the point of impact a several inches to the right at 100 yds. How much is "too much" is probably more a question of personal tolerance than anything else. My own unacceptable limit is 5 clicks. There is a GI service limit for armorers, which I seem to recall is somewhere in the 7 or 8 click range. I think this problem has increased in recent years because of the emphasis on doing barrel installations by clamping the upper reciever. Back when I started fooling w/ ARs, you joined barrels and uppers by clamping the barrel in the vise, in fact the GI manuals specified just that------and I don't ever recall hearing about excessive left windage being needed. Of course, if the upper did twist w/the barrel clamped and the upper free, any point of impact deviation would be to the left, and you'd have to take a lot of right windage.  If you're using a top brand upper and barrel; Colt, Bushie, Rock River, etc; it's very unlikely that any axcessive windage problems come from a misaligned front sight assy----possible, but unlikely. With some of the off brands, anything is possible 
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Thanks for the speedy replies! Shamayim you are right on all counts. Elevation was dead on, but it required 19 clicks of left windage to get things centered. I was using a Brownell's action block, Bushmaster action wrench, and vise on a BM A2 upper with a FNMI marked Gov't profile barrel. Will removing the barrel, and re-torquing the nut help, or am I going to have to replace the upper reciever? Other than the windage the barrel was shooting ~1.5" groups at 50yds once I got it centered using Federal XM193 FMJ. Thanks again! |
| I haad the same problem with my new build .... the front sight was canted. Since the FSB had straight pins and not tapered I removed the fsb and replaced it with a new armalite base. Recentered fsb with rear dead center at the range. Now my rear is perfectly centered as are the groups. |
Assuming the locating pin did dig into the left side of the slot, you're going to have to take the thing apart, and remount it. When you unscrew the collar look closely at the position of the pin in the slot. A magnifying glass helps. If you see any gap on the right side of the pin/slot join, that's what has happened. Remember, we're talking a couple of thousanths here, so that's why the magnifier What I do is clamp the barrel/upper straight up and down so that I can look thru the rear of the upper. I line the whole thing up so that I can look thru the rear, point the assy toward the edge of a doorway or other straight up and down line, center the rear sight, and point the assy so that when I look thru the barrel, that line shows up right in the middle. Then I look thru the sights. In your case, the lined up sights are going to point to the left of of that line. You can try to twist the barrel slightly in the upper by tapping the front sight base with a rubber mallet, to twist it in the upper; but you may have to pull the barrel part way out and reposition it slightly with the right side of that pin bearing against the right side of the channel. When you get the barrel set so that the sight alignment is the same as the barrel alignment, take a set of flat feeler gauges and see if any of them, usually the .002,.003 , or .004 can be fitted into the space between the pin and the slot on the left side. If so, what I do is break off a thin slice of that thickness piece and jam it into that slot; which will keep the assy from twisting back when the collar is tightened. It has to be thin enough so that it doesn't project up into the threads. As you retighten the collar, go gently. My hunch is that a lot of these problems are caused by guys yanking on the wrench when they tighten up the assy. and they're trying to line up the groove for the gas tube to pass thru. I hope the above isn't too confusing. It's really not all that complicated; just hard for me to write out in ways that make sense
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| I had the same problem with an BM M4 upper that a local gunsmith had recently threaded for my preban receiver. It needed 14 clicks to the L to zero at 25 yds. I did not want to drive back to the gunsmith so I attempted to fix it myself. After I reinstalled the barrel with the index pin twisted to the right it was dead on. I did not have an upper receiver block so I had to use aluminum barrel clamps. The main problem I had was the barrel would turn in the clamps after about 40 ft-lbs. Fortunately everything went together at 40 ft-lbsl and the rifle now shoots great. There was no way that I could torque the barrel nut to the "maximum" 80 ft-lbs without slipping. |
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