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Thread: Question about multiple uppers for the same lower

  1. #21
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    Doc, just continue swapping optics and sights around until you shoot them enough to figure out what configurations you like. That's what I'm currently doing with Aimpoint Micros, an NF 2-10x, Leupold 10x and Primary Arms 1-6 along with swapping around muzzle devices and a suppressor. The uppers I'm playing are 10.5", 11.5", 14.5"' and 20" Hbar profile. I had a 16" upper but it was traded away.

    I'll share some observations, if I may. First, I prefer not having an FSB when using an optic. It's not so bad when using an Aimpoint because I can take advantage of the Aimpoint's unlimited eyebox to look over the top of the front sight, whether I'm using 1/3 or absolute co-witness. But the smaller, limited eyebox of the magnified optic means the front sight always blocks the bottom half of the reticle rendering the hold over marks useless. Therefore, any upper that gets dedicated to magnified optic use will not have a fixed front sight. Even when using an Aimpoint, my preference is to not have a fixed front sight.

    Second, heavy optics will make themselves felt. Changing from a 4 ounce Aimpoint Micro to a 26 oz (with mount) LPV will turn a fast handling carbine into a slug. There's no denying that an LPV is more versatile than a non magnified Micro, but do yourself a favor and buy the lightest scope and mount you can afford.

    Third, when using a variety of uppers on a single lower, or when swapping uppers between different lowers, it's important to have everything sorted out. Use a conservative buffer weight (a buffer that falls between an H and rifle buffer in weight) and a quality recoil spring. Either use barrels with the correct port diameters or use an adjustable gas block. Don't use generic extractor springs. I've installed Colt extractor springs in ALL my uppers and have seen a marked improvement in reliability.

    Fourth, I find there is a difference in how a 14.5" upper and a 16" upper handles and shoots. It's subtle and I can't quite put my finger on it, but it's there. Enough that I much prefer the 14.5" upper.

    The only QD mount I'm using is the Scalar Works mount on a T1. I don't trust the durability of the flip lever QD mounts. For the other mounts, I keep a box end, Torx bits and Allen keys in the range bag
    Last edited by MistWolf; 07-12-17 at 10:56.
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  2. #22
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    Quote Originally Posted by Doc Glockster View Post
    Due to the differences in gas port sizes between manufacturers, I think I will stick to all BCM uppers for my BCM lower to try to avoid having to swap out buffers too.

    Are there any technical issues I should anticipate?

    Should I expect all uppers with the same rough specs as the original to be 100% reliable on the shared lower?
    1) All BCM is a good bet, not so much because of gas port sizing but because you are more likely to find all the upper receivers fit the lower. I have had some surprising incompatibilities between top brands, in particular Daniel Defense uppers vs. a Noveske lower. Each of these fit nearly all my other counterparts without issue, so I don't think either one was far out of spec, just at opposite ends of the spec. Even with quality brands nothing is guaranteed, you must test.

    2) Other than the chance of physical incompatibility, no.

    3) No guarantees. However, using an A5 recoil setup with the A5H2 buffer may minimize the chance of issues.

  3. #23
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    Here was my checklist for attempting to maximize upper to lower compatibility. Keep in mind all my bullet points are true for both the upper that came with the rifle and the new upper I purchased from my friend (I helped that friend order the upper in the first place so I know exactly what the specs were). The lower came as a complete BCM rifle with all BCM parts.

    Criteria apply to both uppers:

    1. BCM barrel (to try to get the same gas port size since BCM probably buys their barrels in bulk)
    2. BCM upper receiver (also an item BCM probably buys in bulk, and I know Paul's quality control is excellent so it's probably GTG)
    3. Carbine length
    4. Bravo Company BCG
    5. Government profile barrel (although that probably hasn't got anything to do with reliability other than it might affect how the gun flexes during fire).


    The only major difference between the upper that came with the rifle and the aftermarket upper is the compensator. IIRC Bravo Company did not offer an upper with the standard A2 compensator at the time this was purchased.

    This weekend I plan to test the upper with five or six magazines that have been thoroughly vetted with the original upper. I also plan to use ammo that has been 100% reliable with the rifle as originally purchased.

    If I experience any stoppages, then I will experiment with buffers, springs, and maybe BCG's.

    I honestly expect everything to work right out of the gate since I did my homework and tried to make everything as compatible as possible at the time of purchase.
    Last edited by Doc Safari; 07-12-17 at 13:57.

  4. #24
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    Quote Originally Posted by Kain View Post
    I tried the one lower and multiple uppers thing years ago. All my uppers grew lowers and then produced more stripped lowers which then matured into rifles.

    **** lead farming, I'm a gun farmer.
    The same thing happened to me!


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  5. #25
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    Quote Originally Posted by Kain View Post
    I tried the one lower and multiple uppers thing years ago. All my uppers grew lowers and then produced more stripped lowers which then matured into rifles.

    **** lead farming, I'm a gun farmer.
    Same here. Its a cool idea though.

  6. #26
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    Not to hijack, but I'm working on a similar project just more limited in scope. Looking to build a lower that will run a criterion 18" barreled SPR upper with magnified optics, occasionally suppressed along with a Sionics 11.5 RGP barrel with either an MRO or USO 1-4 with suppressor 50-75%. Lower is currently running an A5H2 on my recce but I'm looking at the new UBR to help balance the weight of the longer barrel on the 18 and suppressor on the 11.5. Currently set-up with SSA-E, but I might swap out for a Tricon - I love the trigger bow! Runs like a standard SSA but I can reduce the 'felt' tigger pull by moving to the flat section for more precision oriented shooting if I want.

    Apologies for the long post, but I've been collecting parts and am almost ready, just looking to see if this is feasible or if I should keep them as separate builds all together as the differences in the function of the rifle and RGP systems wouldn't play that well together on a single lower. Thanks

    Lower will be register as SBR and I have one lower already SBR'd with the A5H2 for function testing.
    Last edited by Glass04; 07-12-17 at 22:16.

  7. #27
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    I never really tried the multiple uppers, but I've switched and swopped out lowers and uppers 25 times with no issues. I've got a bunch of MUR uppers and some of their monolithic stuff, no issues. Stick with the usual suspects Colt BCM LMT etc and you will be fine. Start deviating from the first tier stuff and issues MAY ensue.

    Very strange you got burnt on two Colts, one must have been BLEM PLUS P

    PB
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  8. #28
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    Quote Originally Posted by Pappabear View Post

    Very strange you got burnt on two Colts, one must have been BLEM PLUS P

    PB
    My experience with Colt in the 1990's soured me on them for years. It was a running joke that I couldn't buy a Colt that didn't have a rusted spring on it somewhere. The final straw was a ban era full-length rifle that had an obviously deeply-pitted handguard spring that had been re-blued. I decided Colt was using the civilian market to clean the closets of parts that wouldn't pass milspec. When I returned to AR's a few years ago I still gravitated toward BCM ahead of Colt because of Colt's QC issues. I think Colt is much better now. I have a Colt that shoots just fine and doesn't have any suspect parts that I can tell.

  9. #29
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    I tune my uppers by using a gas port insert or enlarging the gas port so they all use the same buffer. This allows me to be able to swap lowers and uppers but it would allow you to use one lower for multiple uppers.

  10. #30
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    Quote Originally Posted by markm View Post
    I tune all my uppers to run standard buffer systems... with the exception of BCM 14.5 mid length. Those run h buffers best.
    Same for me. I've tried every weight too. Currently running White spring and H buffer. Smoothest naturally aspirated AR I have LOL.
    Last edited by ScottsBad; 07-13-17 at 16:00.

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