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Swapping bolt carrier groups between AR15 uppers

Discussion in 'Rifle Country' started by Parallax, Jun 28, 2016.

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  1. Parallax

    Parallax Member

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    I have a bolt carrier group left over from an upper I sold several years ago. The guy I sold it to didn't want the BCG and it's been sitting in a box in a closet since.

    I'm getting back into ARs and want to order a new upper. Is there any reason I shouldn't just re-use the BCG from the old upper with the new one? I'd rather avoid buying a whole new BCG if possible. They are the same brand (BCM) and I only put about 200 rounds through the old upper with the BCG.
     
  2. G.barnes

    G.barnes Member

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    If it was me I'd use the bcg you have. 200 rds isn't enough to matter.
     
  3. LocoGringo

    LocoGringo Member

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    If it's an entire BCG (bolt, firing pin, carrier, etc.), the only thing I'd worry about is making sure the bolt head spaces correctly to the new barrel. Other than that, nothing else to worry about.
     
  4. ny32182

    ny32182 Member

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    Assuming you are buying a factory headspaced barrel/extension, you should be 100% good to go. Agree 200 rounds on the BCG is not nearly enough to worry about any wear issues.
     
  5. cfullgraf

    cfullgraf Member

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    I like to have the head space checked for any bolt/barrel combination that I assemble.

    While, the design and manufacture of bolts and barrels almost make improper headspace a non-issue, it is easy to check so why not check it and be sure.

    Gauges are not very expensive and last for ever. They are handy to have around. Or, have the head space checked by a competent gunsmith.

    Many folks feel checking the headspace is un-necessary.

    The bolt carrier group will perform just fine in your new project.
     
  6. Grunt

    Grunt Member

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    In .mil land, I put together a LOT of M-16s and M-4s and swapping out barrels is something I can do in my sleep. Out of habit and something I still recommend I always checked headspace just to be sure the weapon is safe to fire. However, out of the literally thousands of barrel changes I've done, I can't remember ever having one that failed a headspace check. But, there's always a first time for everything so I still say, check headspace.
    But, as far as your other carrier group, 200 rounds is nothing so I wouldn't worry about it. Probably the best thing to do with the other bolt carrier group would be to add a barrel, upper receiver, handguards, lower receiver assembly and a buttstock. :evil:
     
  7. MachIVshooter

    MachIVshooter Member

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    +1

    If it were 2,000 rounds rather than 200, I'd swap the bolt out, but still run that carrier. Carriers really don't wear or wear out for a very long time.
     
  8. crashdummy

    crashdummy Member

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    Prior to vacationing in Viet Nam in 1969, the Air Force had everyone take a refresher course on the M16 at Hamilton AFB, CA. They stressed that all parts could be interchanged between rifles, except the bolts. I still follow this practice after 47 yrs of ARs.
     
  9. yugorpk

    yugorpk member

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    That might have been true in 1969. Ive got 7 AR bolts lying around and they are all dimensionally identical (+/- .0001") where it matters which is bolt face to locking lug back. CNC machinery saved the world.
     
  10. Jackal

    Jackal Member

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    200rds is nothing, thats about 30 minutes of gravel pit shooting.
     
  11. MachIVshooter

    MachIVshooter Member

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    We're not talking about the differences between new bolts; it's the wear on the bolt & barrel extension lugs that occurs over time. They wear into each other, mate. Slap a high round count bolt into a different barrel extension, you can end up with insufficient contact that results in excessive stress on lugs.

    As for CNC, all it does is where something like an AR bolt is concerned is increase production. A decent machinist on a good manual machine can hold tenths. Even on my '70s Lagun FTV-2 that has not had the ways and lead screws messed with in God knows how long holds 0.0005" easy. My Hardinge HCT lathe is even tighter. I can machine a perfect AR bolt from bar stock no problem. What I can't do is knock it out in 4 minutes like a high speed millturn.

    CNC is also only as accurate as the machine is tight, including the accuracy of the encoders on the stepper motors, and compensation for backlash.

    Where CNC really pulls away from manual is complex shapes like turbine compressor blades. Can they be done on manual equipment? Yes. Accurately? Yes. Easily? Heck no. The kind of tooling and fixturing it takes to make such parts without CNC is nuts.
     
  12. yugorpk

    yugorpk member

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    As a mechanical engineer who knows what its like trying to hire skilled machinists I'm going to have to defer to the machinery. I could machine a bolt out of bar stock on manual equipment. It would just take me all day to do it. A good machinist could do it in a fraction of that time. Its just finding a good machinist that will maintain tolerances and have two parts that come out identical thats the issue. Hiring people to maintain and feed CNC equipment is a lot easier than hiring people to run manual equipment.

    I used to work in the Seattle metro area. I had to suspend drug testing because I couldnt find anyone that could pass the screening. Those guys were all working for Boeing or their subcontractors.
     
  13. Onmilo

    Onmilo Member

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    I can AND can pass a drug test.
    You hiring?
     
  14. Onmilo

    Onmilo Member

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    I have had brand new M16/AR15 barrels fail headspace simply because they were released for sale without being final finish reamed.
    This has happened with Stainless, Chrome Moly and even one chrome lined chamber barrel.

    How these barrels get through quality control is beyond me but they do.
    I'm sure there are out of spec bolts floating around but because the bolt tolerance spread is quite tight an improperly machined out of spec bolt would be a rare bird indeed.
     
  15. SSN Vet

    SSN Vet Member

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    Build it, and if it chambers factory ammo and locks up properly and the BCG is flush with the back end of the upper receiver, I'd shoot it and smile!

    Unless you're going to build a lot of ARs, I wouldn't purchase headspace gages, as they run ~$30 each for Go and No-go at Brownells.
     
    Last edited: Jun 30, 2016
  16. Ibmikey

    Ibmikey Member

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    I am a small time builder compared to some however I have about a hundred to my credit. I have all the necessary gauges to determine head space but like Grunt have never found an out of spec combination to the point I would reject the rifle. Of course I am referring to new parts (I purchase quality parts...not the cheapest in the pile) and the results are rifles that will reliably spit out empties according to your desires and for as long as you demand.
    With used, particularly well used, bolts I would regulate them to the emergency pile and buy a new one, a good bolt will cost $50-75 bucks and is a excellent investment.
     
  17. 12Bravo20

    12Bravo20 Member

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    The only time that I have seen problems with swapping bolts was while in the Army during basic. Ft Leonard Wood was still using M16A1s for basic training in 1989 and they had very high round counts. I wouldn't worry too much about a bolt/BGC that only has 200 rounds. Like Ibmickey stated, it won't hurt to just buy a new bolt (for piece of mind) and keep the old one for an emergency repair.
     
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