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Thread: M4 FSB

  1. #1
    Runco Gookie frankothebull's Avatar
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    Default M4 FSB

    i'm assembling a new upper for my ar. its flat top with a m4 16" barrel. most of the F type front site blocks i can find to buy are new and have no taper pin holes drilled into them. It seems like it would be a whole lot easier if i could find a FSB with the taper pin holes pre drilled. does anybody have a suggestion on where to buy one with the holes pre drilled? thanks.

  2. #2
    gunco irregular moleman's Avatar
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    You see them for sale on fleabay pretty regularly. If your barrel already has had a front sight on it and was previously drilled for taper pins I'd doubt you'd be lucky enough to get a used fsb with a hole pattern that'll match though. If the barrel hasn't been drilled yet I'd just use a new fsb.

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    Runco Gookie frankothebull's Avatar
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    looks like thats the way im going to have to go. thanks moleman.

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    gunco irregular moleman's Avatar
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    Under the sling swivel area there is a hole from where the gas port is drilled. You can tap that (just the bottem part of the ring) for a set screw to help hold the front sight base in position to double check the front sight position before/while you drill so you don't end up with a canted front sight.

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    Runco Gookie frankothebull's Avatar
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    good tip, sounds alot simpler than the idea i had of building a jig to hold the FSB square to the barrel. the last thing i want is it to be canted.

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    GuncoHolic BBill's Avatar
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    How do you know when you have the hole in the barrel and the hole in the gas block lined up correctly?

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    Runco Gookie frankothebull's Avatar
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    Quote Originally Posted by BBill View Post
    How do you know when you have the hole in the barrel and the hole in the gas block lined up correctly?
    When i was looking at my FSB i noticed that the hole drilled into it looks slightly bigger than the gas hole drilled into the barrel. im assuming that this gives you a little play while lining up the sight.

  8. #8
    gunco irregular moleman's Avatar
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    Also some barrel manufacturers will drill the top of the gas port with a larger size bit (about an 1/8" bit) just enough to get the full bit diameter. That plus the larger size gas port in the fsb gives you some wiggle room. A easy kitchen table gunsmith way to make sure its centered is to remove the upper and set the barreled upper upside down on a flat surface like a formica countertop. The rear of the receiver end will rest on either the sight ears if its an A1/A2 or a small block if its a flat top. The fsb will be the same depending on the type. If the gas block is not centered correctly you can feel an extra bump (kinda like a thud,thud sound) if you rock the upper back and forth. If you make it noticably out of whack its easier to understand what I'm talking about. If its centered correctly it'll seem like a single surface is making contact when you rock the upperback and forth.

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