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AR question/problem

Nwcid
For as much as I know about the AR-15 I just learned something the hard way that I did not know.

I did not know that the FSB was placed in a jig and drilled/tapered "marrying" it to that barrel.

I just picked up a used pistol barrel planning on using a standard FSB only to find out it doesn't work like that [B)] So what do I do now? I don't think it would be cost effective to buy all the tools for something I probably wont be doing again. We don't have a local gunsmith. This is being used on a "cheep" pistol upper build I am doing mostly with parts I had laying around.

My though right now is that since most of the aftermarket gas blocks are just set screwed into place to do that. Is that a bad idea?

I have 2 ideas on how to do that. One is to just tap the existing hold from where the gas port was drilled. Tap that and put in a set screw. Is that enough to keep it in place?

Second thought was to do the above and then drill and tap holes where the pins would go and use set screws there too.

Any thoughts or ideas would be great.
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Comments

8 comments

Date Votes
  • givette
    How you secure the FSB to the barrel is secondary, in my op..and this is a tough call. I personally don't touch them, as it may alter the volume of gas to the point of improper function.

    At Colt, the gas opening in the barrel, the block, and the gas tube volume are all engineered to work with the weight of the BCG and the recoil spring/damper.

    And for a given, known load.

    If you can secure the FSB to the barrel squarely over the barrel port, (using hi-temp loctite as well as the lock-screws), and can get a set of (Wolff?) recoil springs, you will have to perform a range test by swapping out springs until you get functionality, starting from the strongest spring, and working backwards.

    The port total area/volume has to be worked up. As I've said, tough call. Joe


    EDIT-Response to Nwcid (Below)
    What I meant was that I don't mess with gas-block installations. Not at all. Impingement systems have to be (by volume and velocity of the gasses) 'dialed in' by the factory, for total volume, and the placement of just how far the port is from the chamber, all coupled with a particular burn rate of the propellant. That's what I meant by my 'hands-off' policy. Best anyways, Joe
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  • ern98
    Over the last couple of decades I've used more then a few of the aftermarket set screw on FSBs with no real problems or complaints from any of my friends who I've helped with thier rifles.

    But, before you spend a bunch of time, expense and legal hassel making up an AR pistol you might want to see if you can find someone who has one and will let you shoot it. Even the best, carbon fibre, light weight AR pistols, IMO, are of dubious usefullness. A buddy of mine has had one for years and it really is nothing more then an expensive toy/noise maker. Except for the cool factor it won't do anything that his S&W 357 will at pisol distances. At farther ranges it doesn't even do that....
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  • Nwcid
    Joe I understand what you are saying but how is this different then buying say a RRA brand barrel and then adding say a PRI gas block that is held in by set screws? That is a very common practice and you don't hear about a lot of problems.

    EDIT: Thanks Joe.

    ern, I use to have a pistol so I know all about them. I sold it cause I never used it and for the price I was into it was not worth it. With this one I am going to be into it for less then $200 on parts I didn't already have. So at that price just to have one around is no big deal and if I get tired of it I can swap parts around again [:D] Yes I have a pistol lower still.
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  • Mobuck
    I'm not following the statement about "drilling/tapering". If the barrel you have has a stnadard gas block boss, w or w/o tapered pin cuts, another gas block of the same size should fit. The gas port size is a whole different matter that I'm not familiar with.
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  • Nwcid
    quote:Originally posted by Mobuck
    I'm not following the statement about "drilling/tapering". If the barrel you have has a stnadard gas block boss, w or w/o tapered pin cuts, another gas block of the same size should fit. The gas port size is a whole different matter that I'm not familiar with.


    Without the taper pin holes how do you keep the a standard FSB on the gun?

    The FSB fits right over the barrel and lines up with the gas port. That is not a problem.
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  • sandwarrior
    quote:Originally posted by Nwcid
    quote:Originally posted by Mobuck
    I'm not following the statement about "drilling/tapering". If the barrel you have has a stnadard gas block boss, w or w/o tapered pin cuts, another gas block of the same size should fit. The gas port size is a whole different matter that I'm not familiar with.


    Without the taper pin holes how do you keep the a standard FSB on the gun?

    The FSB fits right over the barrel and lines up with the gas port. That is not a problem.





    By using the proper torque on the hold screws for the gas block. Most of the aftermarket gas-blocks I've dealt with don't have the taper pins. A front sight post usually does, although I've come across them without.

    As far as volume goes the FSB has nothing to do with that. It's the .080" hole in the top of the barrel that allows the volume of gas to come through. However, in a pistol length barrel I would reduce that, as pressure is high and therefore more volume with come through it.

    I have been told by another builder that the port size should be .089" standard and .090"-.095" for SBR/pistol length barrels.
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  • v35
    There's not enough material anywhere on the std FSB to hold a set screw.
    You could file the ring bottoms flat(losing the sling swivel)and braze on 1/8" thick or greater steel blocks. That would give you enough metal to drill and tap for setscrews and be able to sock them up tight.
    Loctiting the FSB could prevent future disassembly.
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  • dfletcher
    Two thoughts, neither of which involves using a standard FSB since others have already discussed.

    First, a clamp type 1913 steel base from Model 1 - it never, ever moves and is railed so you can ad a sight that lookls like the standard FSB tower. Second, the Yankee Hill clamp type with a flip up sight. Again, all steel and they don't move. I have them on 4 or 5 ARs including X39 and X51 30 cals. They really stay put.

    BTW, I was of the same thought as you and had the same experience - I figured the standard FSB were pretty much interchangeable until I tried to switch one out. Always something to learn I guess.
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