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  1. #1
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    Install alignment of FSB

    I am building up an A2 upper and it is full of "firsts" for me.

    I have the upper assembled and a 20" Colt barrel waiting on a nut and delta ring.

    I installed the A2 rear sight, which was a bit of a challenge. I had to make my own slotted wood dowel to get the spring compressed on the bottom, so I could get the cross pin through. Went pretty smoothly after I figured out how to make my own tool.

    I don't want to drill on the barrel, so I got a JP clamp on FSB and am trying to figure out the best way to ensure I don't cant it on myself.

    What methods have you used to ensure you don't cant your FSB when installing? Any and all suggestions appreciated.

    Last edited by HKGuns; 10-12-23 at 20:29.

  2. #2
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    Quote Originally Posted by HKGuns View Post
    I am building up an A2 upper and it is full of "firsts" for me.

    I have the upper assembled and a 20" Colt barrel waiting on a nut and delta ring.

    I installed the A2 rear sight, which was a bit of a challenge. I had to make my own slotted wood dowel to get the spring compressed on the bottom, so I could get the cross pin through. Went pretty smoothly after I figured out how to make my own tool.

    I don't want to drill on the barrel, so I got a JP clamp on FSB and am trying to figure out the best way to ensure I don't cant it on myself.

    What methods have you used to ensure you don't cant your FSB when installing? Any and all suggestions appreciated.
    There are jigs for this, but the first thing I’d try would be to put the upper upside down on a table before clamping down the FSB. Then shoot it or boresight it, and see how it goes.

  3. #3
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    Quote Originally Posted by 1168 View Post
    There are jigs for this, but the first thing I’d try would be to put the upper upside down on a table before clamping down the FSB. Then shoot it or boresight it, and see how it goes.
    I'm not sure I'll do another A2, so buying a jig wouldn't be impossible, but not something I'd like to do for a one time use. Didn't know there were jigs for it though, so I'll have to search them out and see how much they go for......

    I plan to align the gas port with a bore scope, maybe that will get me close enough.
    Last edited by HKGuns; 10-12-23 at 20:36.

  4. #4
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    There's an old thread on this. Not sure how much it'll help, but here it is anyway.

    https://www.m4carbine.net/showthread...ig-pics-inside
    “You have made us for yourself, O Lord, and our heart is restless until it rests in you.” -Augustine

  5. #5
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    Quote Originally Posted by HKGuns View Post
    I'm not sure I'll do another A2, so buying a jig wouldn't be impossible, but not something I'd like to do for a one time use. Didn't know there were jigs for it though, so I'll have to search them out and see how much they go for......

    I plan to align the gas port with a bore scope, maybe that will get me close enough.
    One method that won't require a more specialized jig would be to use a dimple jig to dimple the barrel opposite the gas port and then use that dimple to align the FSB.
    “You have made us for yourself, O Lord, and our heart is restless until it rests in you.” -Augustine

  6. #6
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    If I were assembling an A2 upper, I personally wouldn't consider a clamp on FSB. I also wouldn't drill a barrel for an FSB.

    I'd send the barrel to a reputable smith to have the FSB pinned.

  7. #7
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    Quote Originally Posted by HKGuns View Post
    I am building up an A2 upper and it is full of "firsts" for me.

    I have the upper assembled and a 20" Colt barrel waiting on a nut and delta ring.

    I installed the A2 rear sight, which was a bit of a challenge. I had to make my own slotted wood dowel to get the spring compressed on the bottom, so I could get the cross pin through. Went pretty smoothly after I figured out how to make my own tool.

    I don't want to drill on the barrel, so I got a JP clamp on FSB and am trying to figure out the best way to ensure I don't cant it on myself.

    What methods have you used to ensure you don't cant your FSB when installing? Any and all suggestions appreciated.



    Where did you get the A2 sight?
    You won't outvote the corruption.
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  8. #8
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    Since it is a clamp on, you could just sight it in with the front sight adjusting for windage. I did this with a standard fsb that had only one set screw in after removing the sling ring. I kep the handguard off and kept the rear sight dead center, then moved the front to zero it. I will pin the front base of the sight and leave the rear setscrewed with red loctite.

  9. #9
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    Quote Originally Posted by joedirt199 View Post
    Since it is a clamp on, you could just sight it in with the front sight adjusting for windage. I did this with a standard fsb that had only one set screw in after removing the sling ring. I kep the handguard off and kept the rear sight dead center, then moved the front to zero it. I will pin the front base of the sight and leave the rear setscrewed with red loctite.
    That is a very practical suggestion. Thanks.

    I'm giving the extendo Rod a chance. I just ordered the necessary parts off McMaster Carr. Thanks for posting that Prepare.

    EDIT: While I enjoy the process of building this rifle and learning, its quickly turning into a situation where instead of ordering the A2 stripped upper, I should have snagged one of those Colt AR15A4's when they were in stock at Arms unlimited for a great price.
    Last edited by HKGuns; 10-13-23 at 20:39.

  10. #10
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    BRD makes a quality fixture that uses the indexing pin of the barrel to align the FSB against, which should help you perfectly center over the gas port.

    Rumor is Colt would install barrels into the uppers first, then index of the carry handle to install the FSB... which means the sight isn't canted in the least, but also that minor clocking in the receiver means the gas port isn't necessarily perfectly centered under the aperture in the FSB. On a carbine gas lengths system with .062-.070" ports you have plenty of wiggle room... but on .093/.0995" sized rifle length ports, I'd rather index off the gas port/ indexing pin than off the rear sight.
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