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Thread: AR15 Optimal Rail Length Discussion / Fixed FSB

  1. #1

    AR15 Optimal Rail Length Discussion / Fixed FSB

    I found some threads here and on ARFcom discussing this but Iím still confused.

    How do I determine the correct/optimal rail to use on a given upper?

    Back before folding sights, everyone was running a fixed front sight base gas blocks that was connected to the gas tube. And If thatís what youíre using, your gas tube length is determined by the barrel, and I donít think it can be changed. If itís a carbine length gas tube setup, youíre running a 7Ē rail, period. Right?

    Since the invention of low profile gas blocks that can fit under the rail, my assumption is that you want to use the longest possible rail that still gives you access to the flash hider so you can mount a suppressor. This lets you mount more stuff and grab the gun further down. The trade off being the overweight weight is heavier on a longer rail. Is this right?

    Curious to hear if my two assumptions are correct and to help anyone else reading this thread with general info.

    Now onto the specifics of my use case:

    Currently half of my uppers have front site gas blocks and I actually love those. As a survivalist LARPer, the small amount of vision thatís obscured by the fixed front site seems minuscule compared to the benefit of having an always up and durable front site. Also, lately Iíve been switching over to the 2.26Ē aimpoint micro heights and the fixed front site doesnít obscure anything.

    Question) So Iím wondering, if I have a carbine length gas tube system using a fixed sight block currently, on a 11.5Ē barrel, can I install a low profile gas block so I can use longer than a 7Ē rail that feels cramped for a light and laser (running NODs in my LARPing), and then have a gunsmith reinstall the fixed front site on the barrel further down, even though itís not connected to the gas tube anymore, to have the durability of an always-on front site? Would pair great with the always-on built in unity tactical rear sight inside the 2.26Ē aimpoint mount.

    Or am I better off using a backup front iron site that is fixed and mounting that to the rail? Iíve seen those before, seems like it couldnít be as durable as the front site mounted to the barrel, but perhaps itís even more durable since I donít really know how tightly the fixed barrel front site is?

    Question) Whatís the correct appropriate rail to use for the following four barrel lengths: 9Ē, 11.5Ē, 12.5Ē, and 14.5Ē?

  2. #2
    Old man yelling at cloud OlongJohnson's Avatar
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    Quote Originally Posted by Sanch View Post
    Question) So Iím wondering, if I have a carbine length gas tube system using a fixed sight block currently, on a 11.5Ē barrel, can I install a low profile gas block so I can use longer than a 7Ē rail that feels cramped for a light and laser (running NODs in my LARPing), and then have a gunsmith reinstall the fixed front site on the barrel further down, even though itís not connected to the gas tube anymore, to have the durability of an always-on front site?
    Yes.

    I'm putting together such an upper for a .300 BLK using a 16" barrel with mid-length forward block position and carbine gas. You do need the barrel diameters to be compatible with your gas block diameters. May require some custom machining/shimming, etc.
    .
    -----------------------------------------
    Not another dime.

  3. #3
    10" rail on a 11.5 barrel, using a low pro gas block, or a shaved FSB, is just right, IMO. if you need a fixed front, grab a DD. Personally, I'm not prioritizing BUIS, especially if there's a laser on the gun.

  4. #4
    I clung to the fixed FSB for over a decade after I got my first Aimpoint.

    NEVER. Not even one time did I ever need it. Never.

    I finally realized after using 1.93Ē mounts for a couple of years that those buis werenít even good decorations.

    I shaved my last couple FSBs, put on 13Ē handguards, sold the buis on arfcom, and ainít lookiní back.

  5. #5
    Site Supporter rob_s's Avatar
    Join Date
    Nov 2011
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    SE FL
    Sounds like what you want is a "Dissipator" upper if a 16" barrel and a "Kino" upper if a 12.5" barrel.

    all that "gunsmithing" you're talking about is going to lead to problems that are likely more trouble and cost than just buying the right thing to begin with.
    Does the above offend? If you have paid to be here, you can click here to put it in context.

  6. #6
    Site Supporter
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    Sep 2016
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    The 603
    Not an 11.5, but I always thought the BCM 12.5Ē Kino uppers made a lot of sense.
    Ken

    BBI: ...Ēyou better not forget the safe word because shit's about to get weirdĒ...
    revchuck38: ...Ēmo' ammo is mo' betta' unless you're swimming or on fire.Ē

  7. #7
    Site Supporter
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    Feb 2012
    Location
    Lexington, SC
    I'd rather BCM just do a 12.5" Middy upper. I'd buy one today. Actually looking for similar options to replace my current 10.5 LMT in part because I'd prefer a longer rail for ergonomics. At 6'4" a 7" rail with fixed FSB is quite cramped as the OP pointed out. Noveske had one at one time. I think Triarc does also, but I'd be happier with a BCM or Noveske with a low pro FSB and 10 or 11" rail.

  8. #8
    Member
    Join Date
    Jun 2017
    Location
    Colorado Springs
    Quote Originally Posted by Sanch View Post
    I found some threads here and on ARFcom discussing this but Iím still confused.

    How do I determine the correct/optimal rail to use on a given upper?

    Back before folding sights, everyone was running a fixed front sight base gas blocks that was connected to the gas tube. And If thatís what youíre using, your gas tube length is determined by the barrel, and I donít think it can be changed. If itís a carbine length gas tube setup, youíre running a 7Ē rail, period. Right?

    Since the invention of low profile gas blocks that can fit under the rail, my assumption is that you want to use the longest possible rail that still gives you access to the flash hider so you can mount a suppressor. This lets you mount more stuff and grab the gun further down. The trade off being the overweight weight is heavier on a longer rail. Is this right?

    Curious to hear if my two assumptions are correct and to help anyone else reading this thread with general info.

    Now onto the specifics of my use case:

    Currently half of my uppers have front site gas blocks and I actually love those. As a survivalist LARPer, the small amount of vision thatís obscured by the fixed front site seems minuscule compared to the benefit of having an always up and durable front site. Also, lately Iíve been switching over to the 2.26Ē aimpoint micro heights and the fixed front site doesnít obscure anything.

    Question) So Iím wondering, if I have a carbine length gas tube system using a fixed sight block currently, on a 11.5Ē barrel, can I install a low profile gas block so I can use longer than a 7Ē rail that feels cramped for a light and laser (running NODs in my LARPing), and then have a gunsmith reinstall the fixed front site on the barrel further down, even though itís not connected to the gas tube anymore, to have the durability of an always-on front site? Would pair great with the always-on built in unity tactical rear sight inside the 2.26Ē aimpoint mount.

    Or am I better off using a backup front iron site that is fixed and mounting that to the rail? Iíve seen those before, seems like it couldnít be as durable as the front site mounted to the barrel, but perhaps itís even more durable since I donít really know how tightly the fixed barrel front site is?

    Question) Whatís the correct appropriate rail to use for the following four barrel lengths: 9Ē, 11.5Ē, 12.5Ē, and 14.5Ē?
    You're right in your assumptions and rob_s is spot on with the dissipator and kino - both of which have low pro gas blocks and fixed FSPs. I prefer the longest handguards possible for the reasons you mentioned plus another very important one: an extended free float rail will give you better options for supporting the gun as doing it directly on a barrel will affect your POI.

    General consensus is about an inch less than the barrel so 10.3/10.5 barrels get 9.5 rails, 11.5 barrels get 10-10.5 rails, 12.5 barrels get 11.5 rails (there aren't as many of these yet, unfortunately), 14.5 barrels get 13 rails. An important thing to note is the actual length though. Many manufacturers will round down when naming their rails but the length will be slightly longer. For example, BCM has a 13" MCMR rail but the flares on each side are closer to 13.5 I think. There are still plenty of uppers that leave barrels exposed (16 barrels and 13 rails for example) but I think that's dumb.

    As far as what you should do, Centurion makes an MLOK and pic rail cut-out that gives you the best of both worlds and BCM's kino should be solid. Aside from that though, I'd go extended rail with low pro gas block all day. I have enough prepper that I also want BUIS unless I have redundant aiming solutions but I vastly prefer the Scalarworks fixed front over all others because its light and gucci AF.

  9. #9
    Old man yelling at cloud OlongJohnson's Avatar
    Join Date
    Mar 2015
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    Quote Originally Posted by Wake27 View Post
    Many manufacturers will round down when naming their rails but the length will be slightly longer. For example, BCM has a 13" MCMR rail but the flares on each side are closer to 13.5 I think.
    Read the actual manufacturer specs. Last summer, I measured a couple BCM rails of different lengths to project the actual length of a shorter rail of a nominal length would be. Ended up having to sell the barrel I had bought and get a longer one so the muzzle device would clear the rail. Found that if you drill down far enough on the right web site, you actually can find the true, precise lengths for each part number. They are not consistent from nominal length to nominal length, so the only way to know is to get the actual number.


    Quote Originally Posted by Wake27 View Post
    There are still plenty of uppers that leave barrels exposed (16 barrels and 13 rails for example) but I think that's dumb.
    I see no point in hanging more rail weight off the front of the gun than you need rail space to run it and hang stuff.
    .
    -----------------------------------------
    Not another dime.

  10. #10
    Member
    Join Date
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    Colorado Springs
    Quote Originally Posted by OlongJohnson View Post
    Read the actual manufacturer specs. Last summer, I measured a couple BCM rails of different lengths to project the actual length of a shorter rail of a nominal length would be. Ended up having to sell the barrel I had bought and get a longer one so the muzzle device would clear the rail. Found that if you drill down far enough on the right web site, you actually can find the true, precise lengths for each part number. They are not consistent from nominal length to nominal length, so the only way to know is to get the actual number.




    I see no point in hanging more rail weight off the front of the gun than you need rail space to run it and hang stuff.
    My primary purpose for a rail is to cover the barrel. More coverage, more better. Most of the rails out there now a days, youíre talking about mere ounces. The difference between an MCMR 13 and MCMR 15 is legitimately one ounce. I wonít speak for everyone, but Iíd feel like a huge bitch if I was making the compromise of a fully encased barrel for one ounce.


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