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Posted: 12/20/2010 7:38:37 AM EDT
| What are some of the negatives behind an overtightened barrel nut.I am trying to determine if I need to tear down my upper. My rear iron sight had to be adjusted all the way to the left and when I look at the barrel end(unloaded) it is not concentric with the handgaurd. the direction the barrel is of center is consistant with the sight adjustment. One would think that the upper's barrel nut bore was machined out of axis, the threads on the barrel/barrel nut are not alighned with the centerline of the barrel bore or that one of the faces is not perpendicular causing distortion when tightening. Just trying to get some other insights before I tear this thing down. Thanks in advance. |
| What you need: AR15 Upper Receiver Lapping Tool Use this to square the front surface of your upper to the bore axis. I won't torque a barrel without it! |
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It's probably your receiver that's not square. That's my bet also. I can check runouts on the barrel but the reciever is a different story. It would be nice if it was he reciever face where I could skim cut it. I was just curious if anyone had seen this before. It was a cheep lower reciever for a budget build and bets are that I got what was paid for. |
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It's probably your receiver that's not square. That's my bet also. I can check runouts on the barrel but the reciever is a different story. It would be nice if it was he reciever face where I could skim cut it. I was just curious if anyone had seen this before. It was a cheep lower reciever for a budget build and bets are that I got what was paid for. Your barrel shouldn't be attached to the lower receiver. 
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Quoted: Quoted: It's probably your receiver that's not square. That's my bet also. I can check runouts on the barrel but the reciever is a different story. It would be nice if it was he reciever face where I could skim cut it. I was just curious if anyone had seen this before. It was a cheep lower reciever for a budget build and bets are that I got what was paid for. It's the upper that's the problem, not the lower. Use the lapping tool linked to earlier and you should be good to go. |
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Quoted:
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It's probably your receiver that's not square. That's my bet also. I can check runouts on the barrel but the reciever is a different story. It would be nice if it was he reciever face where I could skim cut it. I was just curious if anyone had seen this before. It was a cheep lower reciever for a budget build and bets are that I got what was paid for. It's the upper that's the problem, not the lower. Use the lapping tool linked to earlier and you should be good to go. Where is this lapping tool removing material? Is it where the barrel extension fits inside the upper receiver? Is this lapping done with the barrel removed? It's seems like if the receiver protrusion for the barrel extension isn't square then you're hosed anyway. |
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The lapping tool removes material from the front face of the upper receiver only. It makes certain that the flange on the barrel extension is square with the upper and concentric with the bore. The receiver is lapped prior to barrel installation.
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It's probably your receiver that's not square. That's my bet also. I can check runouts on the barrel but the reciever is a different story. It would be nice if it was he reciever face where I could skim cut it. I was just curious if anyone had seen this before. It was a cheep lower reciever for a budget build and bets are that I got what was paid for. It's the upper that's the problem, not the lower. Use the lapping tool linked to earlier and you should be good to go. Where is this lapping tool removing material? Is it where the barrel extension fits inside the upper receiver? Is this lapping done with the barrel removed? It's seems like if the receiver protrusion for the barrel extension isn't square then you're hosed anyway. |
| You probably got movement in the uppers notch where the index pin fits in..even if theres a milimeter of movement when tightening it will throw the sights off heres how I combat that with a trick I learned from a vietnam era armorer.Cut a rectangualr shim as thick as the index pin and long enough to wrap around the pin when the barrel goes into the upper put the shim behind the pin and use a rubber malet to seat the barrel extension in the upper.Use beer or soda can aluminum for the shim.This will make a snug fit with no movement when its tightened and tourqued.Lapped or not wont matter,if there is movement with the index pin inside the notch wich can and will still happen wether the front of the receiver to extension lip is square or not then the sights will still be off. |
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Josh, Yeah i was thinking about the cheap lower and put thatr in there....did not belong for sure.
John, i'm making a lapping tool at work. What grit lapping compound works. Medium light??? Light???? I'm talking clover compounds here. Pun, I'll have to look closer at that concept. Definately worth looking into. thanks fellas for bouncing ideas of my wall. Fish |
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Definitely fine compound, and proceed slowly, especially at first. If your upper is not square you will notice that the finish on the receiver will come off unevenly at first and this will guide you.
I agree with the comments that others have made about indexing the barrel extension pin in the upper slot, but in my experience this only causes windage errors and canted sight posts. You specifically mentioned that your barrel is non-concentric with your handguard and that makes me think you might have a different problem. Your problem could be beyond what can be corrected with a lapping tool (If your upper is incorrectly machined), but I have never seen this and lapping is a good place to start. Quoted:
Josh, Yeah i was thinking about the cheap lower and put thatr in there....did not belong for sure. John, i'm making a lapping tool at work. What grit lapping compound works. Medium light??? Light???? I'm talking clover compounds here. Pun, I'll have to look closer at that concept. Definately worth looking into. thanks fellas for bouncing ideas of my wall. Fish |
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