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Thread: FSB Install Option

  1. #1
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    FSB Install Option

    I posted a while back about the A2 build I completed and asked some questions about FSB alignment and install etc.....

    The 20" Colt barrel didn't come with an FSB installed.
    - I didn't want to PIN the FSB as I don't have the necessary equipment and I don't want to drill a barrel.
    - I don't want to go to the hassle of shipping it somewhere and have someone pin it as I don't trust anyone to do it correctly.

    Given all of this and that Colt FSB's were hard to find, I decided to go with a JP Clamp on, which went on smoothly.

    I was never really satisfied with the JP clamp on FSB for a couple of reasons.
    - It was adjustable and I don't want an adjustable gas block on this rifle
    - It didn't look right with most all of the other parts being Colt OEM

    I was finally able to source some FSB's and purchased a few with the intention of using one of them on this rifle.

    I looked at options for installation and this is how I decided to install.

    Tools required to modify the FSB: #21 Drill - 10-32 Tap

    I drilled, a #21 Bit, hole through the FSB, then tapped it with a 10-32 tap which is the correct size for Low Profile gas block set screws.

    It came out really nice and seems very secure, only time will tell how this works out and yes, I realize it isn't nearly as secure as pinning.

    The picture below illustrates the final product. One hole you can see, (Pretty Deep) the second hole is under the sling swivel which I removed and re-installed.



    This won't be for everyone, obviously. But hopefully it helps someone out with an option in the future. I am not a machinist, so if I can do it, nearly anyone can.

  2. #2
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    Did you dimple, for the screws to "settle into", the way we do for lo pro gas blocks?

    Because my issue with this would be rotational torque on the FSB from an impact against something, as a slight misalignment would mess with your zero.
    Last edited by ViniVidivici; 02-16-24 at 00:36.

  3. #3
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    Been awhile since I have done it, but the original taper pin holes can be drilled and tapped for at least a #8 screw(and seems like I may have went up to #10) all the way through including the barrel. From there you can put in 4 cap screws of lengths just short of meeting each other and stake them or 2 caps screws or set screws long enough to pass through.

    That originated off of a CMP shooter page that had the gas port hole between the sling swivel ears and all 4 taper pin holes drilled and tapped(either #6 or #8....I went up a size on the overkill above).

    The swivel ear set screw was used to zero for windage with the rear sight set to center and then the other 4 set screws were snugged in.

    I had the set screw only set up for several years of packing around the woods, tossed in the truck seat, or behind the seat and never had it move, but finally ocd got me and I did the threaded through version.
    Last edited by jsbhike; 02-16-24 at 09:42.

  4. #4
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    Quote Originally Posted by ViniVidivici View Post
    Did you dimple, for the screws to "settle into", the way we do for lo pro gas blocks?

    Because my issue with this would be rotational torque on the FSB from an impact against something, as a slight misalignment would mess with your zero.
    I did not dimple as I didn't know if the FSB would align with the dimpling jig I use (KAK) for low profile gas blocks. I thought about it, hard, but decided since the set screws I use have knurling on the ends it would probably be ok. I actually had the FSB in one hand and the jig in the other and my brain was trying to figure out if the holes aligned.

    But you are correct about the FSB rotational force. That thing is a big lever.......

    Since you mention it, I may now go back and dimple as I'll have marks I can use to guide the dimpling.

    Should be fun getting the rocksett out.
    Last edited by HKGuns; 02-16-24 at 09:24.

  5. #5
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    If it works for you OP, then great. I do not like to do things less than thorough. I sent a new barrel and A2 front sight to John Thomas at Retro Arms a couple years ago. Mailed out on a Monday and had it back in hand on Friday that same week. It was $110 with shipping both ways, but an awesome job that was completed super-fast.

  6. #6
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    Quote Originally Posted by ViniVidivici View Post
    Did you dimple, for the screws to "settle into", the way we do for lo pro gas blocks?

    Because my issue with this would be rotational torque on the FSB from an impact against something, as a slight misalignment would mess with your zero.
    I pulled it back apart and sure enough, the mark lined right up with the KAK .450 setting, right under the gas port. So I used the KAK dimple jig and dimpled one of the set screws which should make it more secure and resistant to lateral twisting of the FSB.

    Make sure when you pull gas blocks you don't re-use the set screws as the knurling will not function as it did when they were new.

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