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Posted: 10/4/2014 7:11:37 PM EDT
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got a new DPMS A1 upper receiver and a new 14.5" M4 type barrel assy. thought I properly put them together correctly and everything went just smooth and proper torque on barrel nut. this one went together better than others I have done but I cant get the sights to zero. I'm hitting about 3" to the Right and have all my adjustment used on my rear sight. rear sight cant go anymore, no more adjustment left. I have a new A1 front and rear sight assembly from Fulton Armory. only thing I can think that may be the problem is maybe the indexing on the front threads face of the receiver, the metal is thin in that thread area. or something maybe wrong with my new barrel ? there is some slack when the barrel is in the receiver until the barrel nut is tightened. ? the barrel extension OD is 0.9965 and the OD in the receiver threaded socket part is 1.0030, so I don't know what it may be.
any suggestions would be appreciated.  [font=Times New Roman][/font=Times New Roman]
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Start off by shotgunning the upper open and compare the center of the barrel feed ramps to the center of the front take down barrel. The barrel can slip rotate in the upper receiver barrel socket when the barrel nut is being tightened (causes the FSB to rotate cant with the barrel).
Next is that you can over side torque to bend the upper receiver barrel socket, or even have debris against the socket front lip to back mating surface of barrel extension to cause the barrel bore to be off index of the upper receiver barrel line. Moving on, before you install a barrel, very important to make sure that the FSB has been indexed TDC of barrel with the barrel extension pin (read feed ramps may be centered on the lower lug for a correct install index, but the FSB was not mounted on the barrel TDC instead). As for Flash suppressor, it's is possible to over tighten it, causing the last inch of the barrel to be canted off the center line of the bore. As for bullet strikes to the inside of the FS, then your patterns would be more of a shotgun spray, then tight pattens instead. |
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Armalite makes a "National Match" front sight base that clamps to the barrel. This will allow you to adjust windage with the front sight base so the rear sight is centered in it's movement. You will not be able to mount a bayonet if you use a National Match front sight base.
Who's rear sight are you using? Is it properly seated and tight in the upper receiver? You can square the face of the receiver using Brownell or Sinclair International tools, use this prior to re-installing the barrel.. Slop between the receiver and barrel extension fit can be removed by bedding the barrel to the upper receiver using epoxy. Thinly coat the barrel extension with Devcon epoxy, don't get any on the interior of the receiver before inserting the barrel. If you do epoxy will enter the interior of your receiver ruining your rifle or making it near impossible to fix. Excess epoxy will ooze out of the upper on the barrel side as you slide it in. Wipe it completely clean before re-torqueing the barrel nut. Once this is done you won't be re-barreling this upper again. If you've spun the barrel past the alignment in your upper receiver notch you've sprung the upper and probably ruined it for accuracy work. Your gap between the barrel extension and upper receiver will have increased and your only hope is to try the epoxy option or buy another upper. It may live on as a blaster. I'd replace the upper if it happened to me. |
on the OD measurement
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Well its a DPMS Upper Receiver and didn't cost that much so if I have to get another one it wont hurt so much. the barrel had everything assembled on it when I bought it, Front sight base with pins. I got the Front sight post and rear sight aperture and assembly from Fulton Armory but don't know who actually made it ? The barrel extension is ok, pin and slot in receiver is ok, just a little slack in the pin and slot even when I first slid it in before tightening.
Yes i want to get one of those tools for checking and indexing the seat in the barrel socket anyway. you guys give me some good ideas, THANKS. |
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Contact DPMS and tell them what's going on. They are responsible for properly machining the upper receiver and front sight pin locations on the barrel. If one or the other is off you get this problem. They may look at it and repair/replace as needed. If they didn't make the barrel contact the company that did.
There are some incredibly cheap barrels being sold at this time. Some are under $100. It costs more than that to set up the machinery properly to get a quality barrel. They're scrimping somewhere at that price. |
| I had a new S.S. Barrel Extension laying around, so I checked its fit in the receiver and it fit a lot tighter and the pin looked to be a shade larger too. I think it may be the barrel extension on the barrel or possibly the complete barrel assembly is cheap and just enough smaller tolerances to cause my problem. I don't know who manufactured the barrel but it was a 1-7 twist NATO , chrome lined. for $140.00 and I cant remember the Vender I got it from. |
| Your barrel is probably junk then. You can hardly approach the maker when you can't remember where you got it from. The cheapest barrels I've ever bought were BCM, Rock River and DPMS. All of them are considered major makers and all cost closer to $200 if memory serves me well. They all worked better than expected for what I paid. |
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Before throwing the baby out with the bath water, if the problem is just that the barrel slipped in the upper receiver barrel socket when the barrel nut was being tightened (center of feed ramps is not BDC in the receiver), the barrel can be slipped back in the upper barrel socket without having to loosen the barrel nut.
To do this, pull the upper off the lower, hold the lower across your lap across both legs, with your weak hand, hold the lower receiver against the top of that leg, position the tapper pin installed FSB bare barrrel just past your other leg, then using a non-marring leather mallet, strike the FSB just above the barrel to slip the entire barrel with FSB in the direction needed in the upper barrel socket. If you do this at the range, then with a couple of slips, should be able to get the barrel zeroed with the rear sight dead center. As for if the bottom center of the feed ramps are BDC of the receiver with the barrel installed/ the bore of the barrel is in line with the upper receiver center line, then chances are the FSB was not indexed with the barrel extension pin (why its canted to one side). So here, the tapper pin FSB is removed, and a clamp on FSB that you can adjust on the barrel to correct the windage problem is installled instead. |
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