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With or without barrel nut installed?
I think with a quality barrel nut and proper assembly you'll be fine. Try it and see. An old-timer trick was to fill it up with 242 before torquing but I wouldn't recommend that.
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A lot depends on the purpose of the build. If not for a precision build, then check for play with barrel nut tightened lightly. If no play then when the nut is properly torqued, you should be OK from a functional point of view.
If its a precision build with a barrel that is expected to be very accurate based on reputation, then I would do as you said, and go with a tighter fitting upper receiver.
Another option if you want to salvage this upper is to use a sleeving compound from Loctite, not the common thread lockers, to eliminate the space between barrel extension and upper receiver.
Mark
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Nope, the receiver's not scrap. Once the barrel nut is properly tightened, it'll work fine for your intended purpose.
I'm sure it'll work....but I wouldn't be happy about it. The wiggle certainly will not help accuracy.
I've always had perfect fit with Noveske uppers - no wiggle, but not so tight that it's a struggle getting the barrel on. BCM upper are VERY tight...they recommend you heat the receiver prior to installing the barrel to allow easier installation.
BTW.. BCM has blem uppers marked down 40% for black friday.
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There's no harm in trying what you have; assemble, shoot, evaluate. That's the only way to know for sure with any gun.
Probably a non-issue (considering the platform and the application/intended use). However, if you are worried about it, you could do the following upon assembly:
1. Coat exterior of extension and interior of receiver with Rocksett.
2. Install barrel into upper receiver.
3. Torque down barrel nut, using a good moly grease on the threads (Aero Shell 64, etc.) and torque/loosen a few times before final torque.
4. Wipe off excess Rocksett from inside of receiver.
5. Cure with heat gun.
Aren't there shims you can put around the barrel extension that the High Power people use?
Sure, that's also an option. It's just a bit more of a PITA. If you have easy access to a good set of calipers (capable of both O.D. and I.D. measurements) and various shim stock, then by all means...
You would just want to have a piece wide enough and long enough to cover the entire extension, with only a small gap. I'd orient the gap to the 3 or 9 o'clock position, and cut out a recess for the extension pin to prevent it from spinning while assembling. YMMV...
Wouldn’t it be easier to simply shell out $100 for a new receiver?
I really, really like those BCM blem uppers and have used a bunch. For those unaware: https://www.bravocompanyusa.com/BCM-...-m4%20demo.htm
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