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Posted: 3/5/2011 10:11:02 AM EDT
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Several years back, I built my first AR15 when it was hard to find parts. The rifle fires, but in order for me to zero it, the rear sight is all the way to the left and I had to put a slight bend in the front sight post. I bought the upper on ebay back in the days when you could still get gun parts on there. The upper in an A1 type with an unknown 16" heavy barrel (unmarked). The barrel appeared to be new. This is the only build I have ever had any problems with. I would like to sell this rifle to a friend, but would like to work the bugs out of it first. Anyone know what the issue maybe? |
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Start off by comparing the center of the barrel feed ramps against the front take down block.
If the ramps are not centered on the front take down block, then the barrel slipping in the upper barrel socket when the barrel nut was being tightened, and just needs to be slipped back. Nothing special here (read do not even need to remove the barrel nut, and you just hold the upper receiver alone across your lap, and using a leather non-marring leather mallet, strike the FSB just above the barrel to slip the entire barrel in the socket the needed direction. Now it the feed ramps are centered on the front take down lug, it starts to get tricky. It could be as simple as just the rear sight in use is not sitting true down the receiver/barrel center line. If a removable sight, flip the sight backward and see if the groups go the other way with the rear sight centered out. If that does not cut it, now we get deeper. Here you need to pull the barrel and confirm that the FSB is on the same plain as the barrel extension pin. If it is not, then about the only quick fix it to change out the FSB to one that uses set screws instead of the taper pins. The FSB is jigged up against the barrel extension pin one barrel at a time, then the channels are drilled through both the FSB and the barrel This means that no two FSB 9taper pin channel locations) are identical, and trying to drill a new FSB to true and match the existing taper pins in the channel. If the pin to FSB is OK (on the same plain), then it's time to suspect that the upper receiver face of the barrel extension socket is not true to the center line of the upper receiver, and/or, the entire barrel socket bent off receiver line axis if too much torque as applied. There is a tool that you can use to square up the receiver by hand (hand tools), but I prefer to use a lathe instead. 
Now having said all of this, if you can get the rifle to sight in with the rear sight, even with the rear sight almost all the way over to one side, and with some above problems, then you can still go back to the leather mallet to slightly slip the barrel in the upper socket to rear sight centered zero out the rifle. Granted that the barrel extension feed ramps will not be centered on the front block anymore, you are not rotating the barrel extension so much in the upper receiver that the bolt will have problems entering and exiting the barrel lugs. For such slipping, best to do such at the range where you center out the rear sight, then slip the barrel to perfection so you are windage zeroed out with the rear sight dead center (read unload the rifle, pull the upper, make the blows to slip the barrel, the reassembly the upper and fire another few shots to test again. Note how the barrel slips into the upper receiver socket and only the surface tension of the barrel nut against the front face of the barrel extension holds if fixed into the socket. Due to this, and the fact that the barrel is not threaded into the upper receiver, it can be slipped with the barrel nut still torqued down. 
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Quoted:
Dano, your write up reminded of a question I've had for some time. The gunsmith who built my AR for NMHP said he used red locktite on the barrel etension to increase strength and accuracy. Was this necessary? The rifle has a standard, NON-thick walled upper. Your gunsmith probably read that in the "The AR15 Complete Assembly Guide by Walt Kuleck and Clint McKee" .... I personally think it's complete nonsense to use any loctite when installing a AR15 barrel and both those guys should be kicked in the dingding for publishing that garbage. If the upper is squared like Dano described above and a small amount of the proper moly grease (with no graphite) used on the threads and and the barrel nut torqued properly then there is absolutely no reason to use any loctite. Not to mention I don't think red loctite is not going to withstand the heat produced around the barrel extension and it's eventually going to start leaking out. 
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Quoted:
Dano, your write up reminded of a question I've had for some time. The gunsmith who built my AR for NMHP said he used red locktite on the barrel etension to increase strength and accuracy. Was this necessary? The rifle has a standard, NON-thick walled upper. Totaly different ball game than a standard rifle build. If you are going to glue a barrel in the upper barrel socket, then you better have done not only a lot of pre-work for the upper (like squaring the barrel socket, but you dam near have to jig the barrel as well as the upper receiver so the barrel does not slip and the gas port is no longer TDC of the upper receiver. Also, since the FSB on a match barrel are set screw retained, and not taper pin, to start zero in the windage on the gun, you use the FSB with the rear sight centered. Plus, remember that a match rifle is not going to see enough rounds in rapid secession to even begin to heat the barrel up enough to break down the loctite bond. Now having said this, I have yet to find a match upper and barrel that required loctite since most of the time, I have to polish the barrel extension side walls just so it fits into the socket to begin with. If you are building on a loose combo (BB in a box car and really time to figure which part is out of spec and needs to be replaced), then it may have some merit. As for loctiting the barrel nut to the barrel socket threads, and even the float tube parts together, you betcha, since if you are slung up correctly, you are applying enough pressure to not only put your hand to sleep, but to twist the float tube and it's parts if not glued in place. Remember, the barrel is free floating inside the tube, and it's the CMP hand guard system that is seeing all the torque alone, and why I prefer to loctite them in place for the season. |
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