Ignorance: Why Don't AR-15 Receiver and Front Block Rails Line Up?

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  • HMMurdock

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    This has likely already been asked but I can't find it.

    I'm gonna show my ignorance on a question I feel like I should know the answer to: Why don't the rail on the flat top upper receiver (for the rear flip up sight) and the rail on the front gas block (for the front flip up sight) on my AR-15 upper line up? I'm not a physics professor, but it just seems like it would make more sense if they were on the same plane than being off for a few degrees, no? The rail on the front gas block looks a fair bit shorter in height than the railed upper receiver and I was just curious as to.... why? :dunno:

    Maybe it's an optical illusion but I'm pretty sure the front rail is lower. Any insight is appreciated. :ingo:
     
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    Some of them are just made that way. DPMS that I had was made that way, you can get a front sight that is taller and will put them on the same plain or change out the gas block. I always thought it was a stupid idea too.
     

    natdscott

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    Maybe it's an optical illusion but I'm pretty sure the front rail is lower. Any insight is appreciated. :ingo:

    No, it's real. WHY it was done is a very-damned-good question.

    With irons, you must have the TALL version of any applicable BUIS if you intend to strap it onto the gas block's 1913. This includes the Armalight A2 front, but good luck finding one.

    MOST of the BUIS being made now are much shorter as they are intended to be attached to a railed handguard, whose sections are obviously at the same level as the A4 receiver.

    What are you trying to attach?

    -Nate
     

    HMMurdock

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    No, it's real. WHY it was done is a very-damned-good question.

    With irons, you must have the TALL version of any applicable BUIS if you intend to strap it onto the gas block's 1913. This includes the Armalight A2 front, but good luck finding one.

    MOST of the BUIS being made now are much shorter as they are intended to be attached to a railed handguard, whose sections are obviously at the same level as the A4 receiver.

    What are you trying to attach?

    -Nate

    Thank you; and I thought I was crazy.

    Magpul BUIS on an Alexander Arms .50 Beowulf Entry is what I’m trying to do.
     

    natdscott

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    Generally, your best option here would be to just switch out the gas block, and then have, at your free disposal, the entirety of the market's share of the short front sights.

    Unfortunately, being a .50, your gas block journal is probably something over 0.900"...so almost NONE of the AR-15 blocks will fit.

    Options:

    1) Easiest: get a new handguard with top rail, or MLOK, and mount a short BUIS on that; consider 45-degree mounting.

    2) Find a tall, and STEEL or ALUMINUM**, BUIS that will be okay on your current block. The Armalight is wonderful, but collapse it don't.

    3) Find, or have made, a tall railed gas block that will take the short sights, and still have the journal diameter you need for your .50. If you have to have one made, call Frank White at Compass Lake first.


    **It is my opinion that a plastic sight mounted on a .50 Beo. barrel is asking for trouble. I'd be looking at metal sights from places like JP, Knights Armament, and Griffin. They are tough, and if it is supposed to be a BUIS, it damnedwell needs to be tough.

    -Nate
     

    Hop

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    My RRA 458 SOCOM is like that. A 1/4" rail riser on the gas block solves the problem perfectly. I've since removed the buis and just use a 1.5-4x scope but the irons worked fine with the riser.
     

    JJFII

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    I didnt read all the answers but I can explain why.

    There are two heights above bore because there are two ways to need a BUIS. An A2 upper may or may not have a fixed FSB. Some Gas blocks were made low to allow the folded FSB to be flush with a handguard. This required the FSB to be taller when unfolded. Now when you place a full length railed handguard which is the same height as a Flat top, you now need a shorter BUIS in front, so they made a second type of FSB (shorter) to accommodate the new trend of full length railed handguards.

    At one time all this full length stuff was not common. Everyone had either an A2 or flat top with the standard handguards. The first thing to get modded was the gas block...and that is where all the fun started.
     

    natdscott

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    I alluded to that...I was just hoping there was some extraneous reason I hadn't heard. Apparently not.
     

    russc2542

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    Overthinking much?
    So you can remove the free-float handguard without having to remove the gas block. Think about the milions of cheap ARs being sold these days and marketing/convenience needs of the masses.

    Having the F BUIS below the rail makes sense too so it won't block the view from a low-mounted scope... except you wouldn't be able to use it unless you removed the scope, which is doable but meh.
     

    natdscott

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    Overthinking much?
    So you can remove the free-float handguard without having to remove the gas block. Think about the milions of cheap ARs being sold these days and marketing/convenience needs of the masses.

    1) Those gas blocks were in use long before the current craze of handguard-of-the-week. An A2, or similar, set of guards, with or without float tube underneath, cannot be take off without removal of the gas block.

    Even some of the current railed lo-pro guards require a MUCH smaller gas block than anything with a rail. Centurion is one I've worked with, and there are only a few gas blocks that would fit with those, and similar.

    You may still be correct about the provenance of the low railed gas blocks, and the logic makes sense, but I think the time frame is off.

    Having the F BUIS below the rail makes sense too so it won't block the view from a low-mounted scope... except you wouldn't be able to use it unless you removed the scope, which is doable but meh.

    2) The gas block being equal to, or below the rail height of the A4 receiver is irrelevant to use of any sighting system I am aware of. It just doesn't get in the way of any sight higher than a Marble front bead.

    Can you SEE it on 1x? Yeah, but then, you can see the barrel as well. Neither one is in front of the reticle.

    -Nate
     

    Hohn

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    I don't know how the "low" gas block rail ever came into existence, it's so much lower that it cannot be explained by the F-marked vs standard A2 FSB.

    I don't think the hand guard-swap theory explains it. Most hand guards won't go over even the lower railed block.
     
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