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Thread: FSB cant spec

  1. #1
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    FSB cant spec

    Just a quick question. I bought a used DD M4V3 a little over a year ago. The guy had replaced the FSB with a low pro gas block.

    Anyway, I like keeping my weapons stock, so I had a local gunsmith put the original FSB back on. It was the original, so the pin holes should line up.

    A few months ago I was checking it out, and the front sight post is about 1/16" off of the centerline of the bore.

    Is this within spec? The rear BUIS is cranked over 3/4 of the way to the left, but it isn't bottomed out, and I have a good 50m zero with it. I am only concerned that it may be off at extended range. I never tested it though, cause I just always checked my BUIS zero and shoot with my ACOG.

    Thanks!

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    Quote Originally Posted by Bolverk93 View Post
    Just a quick question. I bought a used DD M4V3 a little over a year ago. The guy had replaced the FSB with a low pro gas block.

    Anyway, I like keeping my weapons stock, so I had a local gunsmith put the original FSB back on. It was the original, so the pin holes should line up.

    A few months ago I was checking it out, and the front sight post is about 1/16" off of the centerline of the bore.

    Is this within spec? The rear BUIS is cranked over 3/4 of the way to the left, but it isn't bottomed out, and I have a good 50m zero with it. I am only concerned that it may be off at extended range. I never tested it though, cause I just always checked my BUIS zero and shoot with my ACOG.

    Thanks!
    First, make sure that the pins in the FSB are full seated. This directly affects the FSB's position.

    Second, the rear sights can be something like 15 clicks one way or another to be "in spec."



    C4

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    Quote Originally Posted by Bolverk93 View Post
    A few months ago I was checking it out, and the front sight post is about 1/16" off of the centerline of the bore.
    This is impossible. Do you mean the front sight post is 1/16 off of top dead center?

    You can easily check this out by trying to get a plain old 100 yard zero.
    "You people have too much time on your hands." - scottryan

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    Years ago I contacted a major firearm manufacture about the necessity of putting 13 clicks left windage on my rear sight to zero my brand new AR-15 at 100 yards. I was advised that 13 clicks was within the acceptable range, which was unacceptable to me.

    I noticed the front sight base was canted to the left a few degrees, which would make my shots go right if I set the rear windage knob dead center at zero. If I had to compensate for left to right wind at 300 yards or more, I did not have enough travel on my rear sight to get on target on a windy day. I reindexed the barrel to correct the windage issue. I have done this on several AR-15s to achieve zero windage on the rear sight on a no wind day. If your front sight base is only off a few degrees, you may be able to do the same thing. Find someone with the tools and know how to help you do this.
    Last edited by 7 RING; 01-04-13 at 10:17.

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    By re-indexing the barrel; does that mean you loosened the barrel nut and rotated the barrel within the limits of the pin fitting into the reciever?

    How does that affect the alignment of the feedramps?

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    Quote Originally Posted by Bolverk93 View Post
    By re-indexing the barrel; does that mean you loosened the barrel nut and rotated the barrel within the limits of the pin fitting into the reciever?

    How does that affect the alignment of the feedramps?
    Yes, and the feed ramps would also move. Make sure the fsb is secured properly first. You can also remove a little material from the notch in the receiver if its needed to bias the fsb a little more.

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    Quote Originally Posted by Bolverk93 View Post
    By re-indexing the barrel; does that mean you loosened the barrel nut and rotated the barrel within the limits of the pin fitting into the reciever?

    How does that affect the alignment of the feedramps?
    As stated above, the feed ramps on the barrel extension will rotate with the barrel.

    Most of the barrels I reindexed only required
    1) Loosening the barrel nut (this will require removal of the gas tube)
    2) Placing a wood dowel rod inside the front sight base
    3) Applying pressure in the opposite direction the barrel nut tightens
    4) Tightening the barrel nut to proper torque

    What little slack there was between the index pin and barrel nut usually compensated for 10-12 clicks of windage adjustment. Only two of the rifles I worked on over the years required removing any material from the index pin and it was only a few thousandths of an inch. If your windage is within 3-4 clicks of zero, I would not remove any material.

    After you retorque the barrel nut, turn the upper receiver assembly upside down and make sure the gas tube appears centered in the area where the charging handle rides in the upper receiver. On most of the rifles I worked on the gas tube appeared to be more toward the center of the upper receiver than before I reindexed the barrel.

    Most rifles are a mass produced item and it is not unusual to find one that requires this procedure once in a while.

    Good luck and let us know how this works out.
    Last edited by 7 RING; 01-04-13 at 16:33.

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    I would NEVER remove material from the upper receiver.

    Quote Originally Posted by tehpwnag3 View Post
    Yes, and the feed ramps would also move. Make sure the fsb is secured properly first. You can also remove a little material from the notch in the receiver if its needed to bias the fsb a little more.



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    Quote Originally Posted by Iraqgunz View Post
    I would NEVER remove material from the upper receiver.
    Agreed. That some bootleg ass PatrickSweeney level advice.
    "You people have too much time on your hands." - scottryan

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    Quote Originally Posted by Iraqgunz View Post
    I would NEVER remove material from the upper receiver.
    I wouldn't either, if I didn't have to. Its been done before on less perfect assemblies to get the windage centered. But I'm sure I don't have to tell you that, though.

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