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Posted: 8/24/2012 9:51:01 AM EDT
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I just got through building my first flat top AR, and am having trouble sighting in. I had to move my flip up rear sight nearly all the way over to one side to zero in at 25 yards. I mounted a new un-adjusted scope, put the gun in a vice, and lined up the bore with a large black circle on a sheet of paper 10 yards away, then checked the scope cross hairs. The cross hairs were lined at least 12" left of the black circle. When looking through the scope, the front sight tower is at least 1/4" to 1/2" to the right of the vertical crosshair. It's not canted to the right, It looks straight vertical but just shifted to the right.
I looked at where the barrel extension/upper junction, but can't see anything obviously out of alignment. I also checked the receiver for square with a small steel square, and checked the attachment between the barrel and barrel extension, but everything looks good. I sighted down the barrel, and it looks perfectly straight. My gunsmith charges $80 to put everything in a jig to check alignment. If it's likely the receiver, I'd rather just buy an new one than spend $80 to find out what I already suspect. My questions are: Could anything be out of alignment enough to cause this problem and still not be noticeable to the naked eye? Is it more likely to be an out of square receiver or barrel/barrel extension problem? Would lapping the receiver be enough to correct this much of misalignment? Any of your insight or advice would be appreciated. |
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Start off by just looking at the center of the barrel feed ramps to the center of the front take down lube below it.
If the barrel slipped/rotated as the barrel nut was being tightened, then your going to see it there first. Granted that the extended barrel ramps help tell the tail that the barrel slipped in this receiver, you should be able to tell even without barrel ramps in the upper, 
If the center of the feed lips are centered on the front take down lug, then it's time to pull the barrel and now check to make sure that you did not get any debris between the end of the upper receiver barrel socket and the inner edge of the barrel extension lip. Also, with the barrel in hand, confirm that the FSB is on the same index as the barrel receiver extension. Also, if you have a lathe handy, you can spin the barrel to see if the barrel is warped or bent as well. Without a lathe, it can still be done, but a tad more trickier. Not a fan of hand lapping with a drill to square the barrel face, but if that is your only option, then so be it. Myself, prefer to just Mandel the upper (pressed in so it will square the socket itself for starters if someone has slightly warped/canted the upper barrel socket via over torque'g), then lathe face the upper receiver socket instead. Mandel pressed into the upper, upper in the lathe, and a first light cut to see just how bad the upper face is not square with the center line of the receiver, 
Second pass, and have backed off the live end to get a better look/take the photo only since all cuts will be with the live end in play. Not bad, but still needs about .003 more removed to finish off the face. 
Face squared/clean with the center line of the receiver, and ready to install the barrel. 
As for just replacing the upper receiver, could just end up the same place you are now, with the new upper barrel socket not true with the center line of the receiver either. |
| I used a mil-spec 3-pin barrel wrench on the end of a click type torque wrench. I ended up with about 45# torque with the gas tube hole lined up. I inspected the mating surfaces between the barrel extension and receiver, and both were smooth and clean. The index pin on the extension and center of the gas block line up perfectly, so I'm sure the pin wasn't disturbed. I've just about convinced myself the upper receiver is out of whack, even though it's not apparent to the eye. |
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Disassemble the upper, make sure there is no foreign debris or grease on the bearing surfaces of the receiver, barrel and nut. The barrel should fit snugly into the receiver. The nut should be torqued and loosened 3 times before the final setting. Make sure the threads are lubricated with light grease or oil. Dry threads will cause false torque readings, galling or seizure. After reassembly, stick a long straight cleaning rod down the barrel from the muzzle until it extends past the rear of the receiver. The rod should be in the center of the receiver. *edit* with the barrel and gas block removed, you can roll it on a glass or marble counter-top to make sure it's straight. Also check to see if the bore is concentric to the barrel. |
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Quoted:
I used a mil-spec 3-pin barrel wrench on the end of a click type torque wrench. I ended up with about 45# torque with the gas tube hole lined up. I inspected the mating surfaces between the barrel extension and receiver, and both were smooth and clean. The index pin on the extension and center of the gas block line up perfectly, so I'm sure the pin wasn't disturbed. I've just about convinced myself the upper receiver is out of whack, even though it's not apparent to the eye. it doesnt matter how well they line up beforehand, a lot of times if you use barrel jaws in a vice and torque the barrel nut it will wallow out the square hole in the upper receiver and cause the barrel to twist. that is why the jaws that go over the upper and lock into a vice work so much better. |
| I didn't use barrel vice jaws. I used an upper receiver action block that goes over the upper receiver, then clamps in a vice. I also used the insert that goes inside the bolt/carrier channel. I did everything by the book, and the slot the upper receiver extension pin fits in is undamaged. In fact, it's a pretty tight fit. |
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So if you have weeded out that the barrel extension feed ramps are indexed correctly, the FSB is indexed correctly to the barrel extension pin, and the barrel is not warped, then the problem is the barrel socket is not true to the center line of the receiver.
So at this point, it either correct the barrel socket to get it on face, or just slip or reindex the barrel in the upper receiver socket to get the rifle to zero out with the rear sight centered. Yes, the feed ramps will not longer be index correctly, but with the slight amount of cant that you have to add to the barrel now, should not cause bolt to barrel extension lug lock/unlock problems. |
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Quoted:
So if you have weeded out that the barrel extension feed ramps are indexed correctly, the FSB is indexed correctly to the barrel extension pin, and the barrel is not warped, then the problem is the barrel socket is not true to the center line of the receiver. So at this point, it either correct the barrel socket to get it on face, or just slip or reindex the barrel in the upper receiver socket to get the rifle to zero out with the rear sight centered. Yes, the feed ramps will not longer be index correctly, but with the slight amount of cant that you have to add to the barrel now, should not cause bolt to barrel extension lug lock/unlock problems. I'm almost on board with truing the receiver (facing the barrel socket). Wouldn't it be necessary to verify that the collar on the barrel extension and the barrel extension itself are perpendicular and true to the bore axis? |
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Quoted:
Quoted:
So if you have weeded out that the barrel extension feed ramps are indexed correctly, the FSB is indexed correctly to the barrel extension pin, and the barrel is not warped, then the problem is the barrel socket is not true to the center line of the receiver. So at this point, it either correct the barrel socket to get it on face, or just slip or reindex the barrel in the upper receiver socket to get the rifle to zero out with the rear sight centered. Yes, the feed ramps will not longer be index correctly, but with the slight amount of cant that you have to add to the barrel now, should not cause bolt to barrel extension lug lock/unlock problems. I'm almost on board with truing the receiver (facing the barrel socket). Wouldn't it be necessary to verify that the collar on the barrel extension and the barrel extension itself are perpendicular and true to the bore axis? See the Mandel pressed into the upper receiver (silver large thing inside both the receiver and barrel socket),
My way, when the Mandel is pressed in, it puts everything on the same center line to true a socket that is slightly out of center line. With a drill type facing tool, since the tool has to be able to spin inside the receiver/barrel socket, then really, it only trues the face of the upper barrel socket to itself for the most part. Again, the tool has to be able to turn, so there is some slop that has to be accounted for when even doing this. So on that note, if you don't have a lathe, then the drill type tool does have some merit, but I don't consider is the best way about going to solve the problem, especially when a upper socket has been slightly tool torque canted off center line when a barrel was being installed. |
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