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Posted: 6/1/2011 6:23:06 PM EDT
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After assembling a YHM Free float handguard onto my friends bushmaster, we noticed the barrel flexed up and down a little... thought it was no big deal because they all do that to some degree, but this seemed a little more than usual.
When at the range, the rifle functioned fine, but when we set the barrel on the sandbags, we got an eight inch higher point of impact than when we set the rifle on its bipod attached to the handguard... I expected a little bit of difference, but not eight inches... What is SHTF and he has to shoot out his window? he would have to expose himself an inch higher to shoot it off the handguard than the barrel? You can visibily see the barrel flexing, up. It was torqued to over 50 pounds until the hole lined up, I can't torque it any more. Any ideas? |
| Was the barrel a loose fit in the upper? If so you might try two things. First, square the front of the upper by lapping. That will make sure the back of the barrel extension is fully against the front of the threads. If the front of the threads is a lot out of square, and I have seen quite a few that were, that can cause a pivoting affect no matter how tight the barrel nut is tightened. You can also try bedding the barrel extension into the upper. I have heard about using everything from Accra-Glas gel to Lock-tite to JB Weld but have never found it necessary to go that that extreme myself. Personally if I were doing that I would use Accra-Glas gel because it is very easy to use and if need be the bond can be broken by a heat gun and you won't have to use something as hot as a torch like you might with Lock-tite or JB Weld. AG gel might not hold up to crazy overheating caused by FA fire but it will hold up to anything a conservative shooter will dish out. |
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OK forgive my ignorance, what is "lapping"... laymans terms, I barely change my own oil...
Thanks for the advice! Yes the barrel is a loose fit in the reciever... I held it in tight with all my might and then hand tightened the barrel nut with the wrench, then torqued it and it seems to have made a slight difference... |
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If the barrel is actually loose in the upper after torquing the barrel nut to spec (i.e. you can feel it moving when you push on it) then you have a problem. If its just a zero shift when resting the barrel on stuff, then stop resting the barrel on stuff. The whole point of a free-float tube is to isolate the barrel from external influences. If you have to rest on a window, rest the free-float tube on it, not the barrel. The barrel should not rest on anything. If you do rest the barrel on anything, you should not be surprised if it affects your point of impact.
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Quoted:
If the barrel is actually loose in the upper after torquing the barrel nut to spec (i.e. you can feel it moving when you push on it) then you have a problem. If its just a zero shift when resting the barrel on stuff, then stop resting the barrel on stuff. The whole point of a free-float tube is to isolate the barrel from external influences. If you have to rest on a window, rest the free-float tube on it, not the barrel. The barrel should not rest on anything. If you do rest the barrel on anything, you should not be surprised if it affects your point of impact. This. |
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Lapping is a process by which you "square" the upper receiver where the barrel extension fits in. Get a lapping tool and some compound.
The tool fits in the upper, just like the barrel would then spins to "sand" off any high points. Lapp the upper and bed the barrel and you should be able to remove most if not all of the slop. You should not be able to move the barrel once it is torqued on. |
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Quoted:
Even 8+ inches difference? I expected 2-3 inch difference... but 8+? Yeah we agreed not to rest it on the barrel, but youcan feel that it is loose up and down and side to side still... You don't stipulate the distance you were shooting; resting the barrel on something could shift POI by 8" at distance (say 200 yards +). However, if the barrel is so loose in the upper receiver that it can be visibly moved by hand pressure, then clearly there is something wrong. Lapping may help, but I would suspect something more serious like an out of spec receiver, barrel nut or barrel. If you have a known-good upper you could take some measurements off that for comparison, or you could switch parts until you identify the culprit. |
| More than likely the barrel nut has came loose due to the fact that whoever assembled it could not get the notch in the nut to index correctly for the gas tube and be within spec torque values and instead of lapping the upper to correct it they just backed it off some to index the notch. |
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He clearly stated that they torqued the barrel to 50 pounds/until the next gas tube hole came into alignment, so he has enough torque on the barrel so tightening it is not the problem. No barrel nut can loosen up that much from 50 ft-lbs and the gas tube was installed.
One concern I have is that the upper cracked even though he did not use excessive force on the barrel nut. That would be a manufacturing defect in the upper if only 50 ft-lbs cracked it. Only way to tell that though is to remove the barrel but since lapping is not going to hurt it anyway and may well cure it if that is all the problem is, the barrel has to come off anyway. I've seen uppers quite out of square but I don't think even the worst upper I've seen would cause the barrel to be that loose. The more I think about it the more I fear the upper cracked. I have heard of one YHM barrel nut cracking a few years ago, so if anything cracked let's hope it was the barrel nut and YHM will make it right if the nut cracked. |
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Quoted:
He clearly stated that they torqued the barrel to 50 pounds/until the next gas tube hole came into alignment, so he has enough torque on the barrel so tightening it is not the problem. I should know better than to skim read these threads ....sorry.... carry on. 
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