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  1. #1
    I am a Forum User
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    Gas block and tube question for the experts

    In a dilemma. I just purchased a used upper from a buddy in 450 legend, missing the gas block and tubing. I have a bca 350 upper I've been using for reference. On my 350, the tube seems to measure about 11.75" from end of gas block to the bolt carrier end.

    My question is, I know I need a low profile gas block because of the forearm. The 450 upper measure's 18.75" from end of barrel (before muzzle break) to the bolt carrier mark. Where the old gas block was is the same measurement as my 350 upper.

    Can anyone tell me the exact setup I should buy (length, diameter, which block)

    I'm brand new to the platform, thanks for the help.

  2. #2
    I am a Forum User
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    I'm not an expert, but have tinkered with a few arfs.

    This all depends on which gas system you have on your upper.
    From the chamber to the hole where the gas block goes as in this diagram.

    More reading on the topic at Primary Arms: https://www.primaryarms.com/MCategor...-15-Gas-Blocks
    (top of the page, click on Gas Block buyers guide)

    Measure that, and order the correct tube.
    Also measure the journal where the block attaches. Most common in the ar15 works in 750, but not all are the same. Different barrel profiles will have different journal sizes.
    Save a good amount of brain damage and buy an adjustable gas block. Most systems are by default overgassed, and this allows you some flexibility when playing with different ammo, buffer weights, barrels, systems, etc. Spend the extra couple bucks for an adjustable GB.

  3. #3
    MGO Member Moleman-'s Avatar
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    "450 Legend" did you mean 450 Bushmaster? Sounds like you need a Mid-Length gas tube which is 11.75" long. Rifle tube is about 15-1/8", carbine is 9-3/4", and pistol is usually 6-3/4". 350L and 450BM GENERALLY will run better with a carbine length gas system even if the barrel is longer than 16" as the available port pressure is less than a 223/5.56. Since you already have a mid length gas system on it, I wouldn't bother with an adjustable gas block and would just get a decent quality steel block. YHM sells a good quality low profile gas block that isn't expensive. Lately I've been getting my gas blocks from JSE surplus who has decent quality nitride finished gas blocks for $16 or $20 (and gas tubes for $10 or $15 for nitride). As Quads mentioned you'll need to measure the gas block journal on the barrel. It is likely .750" but could also be .875" or if a bull profile possibly even .936". I prefer to make mine with a .875" gas block and 11/16"-24. muzzle threads.

    What does the gas port dia measure (not the beveled top section if present)?

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