AR15.Com Archives
 AR-15 barrel problem
JamesP81  [Member]
8/20/2011 7:14:13 AM EDT
Some of you may have seen my other two threads dealing with zeroing issues of my new Sabre Defence upper. I have tracked the problem to two sources. One is that my ACOG is a piece of junk that won't hold zero (going back to the factory). The other is that my FSB is canted far enough to the right that I don't have enough windage adjustment in my rear sight to dial in my shots at 25 yards.

I'm interested in the canted sight issue. You can't very well redrill the pin holes in the barrel, and even if you could, I don't have the tooling for that. That means I have to replace defective parts. Clearly, the culprit is either the barrel or the upper receiver housing. Before I start throwing money at the problem, I'd like to know if there is a way to measure and determine which part is defective. I can see how if the slot for the barrel in the upper was cut wrong, you could end up with a canted FSB just as easily as if the FSB pin holes were not drilled properly. Is there any way I can measure or check the upper to see if it's the problem? I really don't want to buy a barrel and have the same problem over again.
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Dano523  [Team Member]
8/20/2011 11:28:55 AM EDT
With the barrel off the upper and the FSB pulled, check the indexing of the gas port to the barrel extension pin. They should be both top dead center. If they are not, consider the barrel trashed.

If both are TDC, then reinstall the FSB and now check both for the same TDC plain as well. If the FSB with taper pins is not indexing TDC, then forgo using a taper pin unit, and instead replace the FSB with a set screw/clamp on type.

Now chuck the barrel up in a lathe and check if the barrel is bent with a indicator. If only slightly bent, it can be straighten. If the end of barrel section threads are bent. then consider the barrel trashed (at least that part of it).

If the barrel is not bent, then suspect that you have a problem in the upper receiver barrel socket, and it may need to be faced to square it back to center line of the upper receiver.

Now with the above done, reinstall the barrel, and bore sight with the rear sight centered (making sure to re-index the barrel in the socket if the barrel twisted when the barrel nut was tightened.

Lastly, when you install the FS, make sure to not over torque it or you will end the end of the barrel, and check to confirm that the FS bore is concentric with the barrel bore so you don't get bullet strikes.
Mace_USMC  [Member]
8/21/2011 7:54:46 AM EDT
Something I've seen once or twice on a bad upper was the feed ramps not lining up, so that's something you can look at real quick. Also mark up your bolt and barrel extension with dykem or sharpie run the bolt back and forth and see if the lugs are wearing strangely or anything and that can tell you if the index pin slot is off center or anything. Another thing that might clear up your problem is squaring up the front of the receiver face, it might be out of square and that can push your windage off to a noticeable level.
JamesP81  [Member]
8/21/2011 4:13:39 PM EDT
I believe I have discovered the problem. While I had the FSB off, I re-installed one of the taper pins into its slot:

This would seem to be a problem. It cannot be fixed, so if I want to use irons, my choices are to install a clamp on gas block with a rail, or lo-pro clamp on and a FF rail with a detachable front sight, or rebarrel. I couldn't really tell with the FSB installed, but the direction the pin is slanted will obviously cant the FSB over to the right, which explains why the rifle is shooting so far to the left.

Also, the index pin slot might be ever so slightly to the right of where it should be; I can't really tell for certain. I am certain that if it is off, it's not by so much that it couldn't be adjusted out with a few clicks of the rear sight. The above problem, however, is pretty much intractable.

I have concerns about using a clamp on gas block for this rifle. It just doesn't seem secure. Despite the cost involved, I am inclined to rebarrel, but I haven't decided yet.
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Rino451  [Team Member]
8/21/2011 5:02:59 PM EDT
Hell, if you're going to rebarrel, you might as well go on and get a new upper and barrel. If $'s are an issue, I'd just get a clamp on FSB and an upper. Costs under $150 - less than a barrel - and solves all the symptoms/issues.

gutermung919  [Member]
8/22/2011 9:14:01 PM EDT
even tho the taper pins are off, it doesnt automatically mean that the barrel is bad. Check the gas hole in the barrel and make sure that it is in fact TDC. It might just be that the barrel needs a slight tightening. I see it all the time on military weapons.
Redtazdog  [Team Member]
9/3/2011 12:46:02 AM EDT
Call Sabre Defence and see what they say about the gunsmith that was drunk at the wheel when drilling the sight pin holes.
Maybe they will help you with the problem

NickySantoro  [Member]
9/4/2011 7:14:54 AM EDT
I had the same problem with an FN A2 barrel I got at a gun show. Same issue with windage. After knocking out the taper pins I found two holes for each pin. Neither was correctly located. After making sure I had the FSB dead center and had the gas ports aligned, I drilled straight through with a slightly larger diameter drill bit, then tapped the holes. Four cap screws, two coming in from each side and ground down in length so that they just touched in the middle when tightened down, secured the FSB. Not an elegant solution but it's held for a dozen or so years and cost about a dollar for the four cap screws. I should have used set screws for a more finished look, but I had the cap screws and it is just a beater rifle anyway with the bayo lug ground off.
GunCat  [Team Member]
9/4/2011 3:57:01 PM EDT
Originally Posted By Redtazdog:
Call Sabre Defence and see what they say about the gunsmith that was drunk at the wheel when drilling the sight pin holes.
Maybe they will help you with the problem

Might be tough to find Sabre's phone number...

A clamp on front sight - or NickySantoro's set screw idea will take care of the issue.
1MAC  [Team Member]
9/5/2011 3:13:12 PM EDT
John Holliger at White Oak installs FSB's with opposing set screws and zeroes for windage for $50.

White Oak Precision

Call him or email him and see if he'll do it on a rifle that's already been pinned. If he does it, it will be done right.
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