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Posted: 10/7/2013 8:30:14 AM EDT
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Hello Build Forum,
Recently completed my first build using a lower from 80percentarms.com and an AR150 kit from Sarco . I did this mostly for a learning experience. The lower matches up to the upper ok, but the fit is a little sloppy. The gun fires just fine though. I also tried matching the lower to a Colt upper and it matches up just fine, nice and tight, but the bolt won't retract into the buffer tube. So, before I go any further, I'd like to know what to look for regarding compatibility when selecting an upper receiver? Or if anyone has any comments regarding my Colt issue? I've ordered another 80 percent lower from jamesmadisontactical.com so my first order of business is to find an upper that I'm sure will work with it. I'm also interested in an comments on upper/lower compatibility in general. Thanks for your comments, Rick |
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If the takedown (rear) pin is drilled too high you could close the rifle (and be sloppy) but the bolt would be too high to move back into the extension.
If that's the case then the lower is out of spec. Mil spec by definition means that it all fits together regardless of who made it. Since it works on the rifle, keep them together. 
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Some upper receivers will be super tight and some lowers will likewise be the same...Once you get both upper and lower with very tight
tolerances you will sometimes get fitment problems or you will need to make adjustments to get everything to fit correctly... I have noticed for example Aero Precision uppers come with very tight tolerances and often have to be fitted to the lower receiver as a result...this is also true with 80% completed lower receivers. The best part of that equation is you will get a more accurate fit and better accuracy etc. |
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Quoted:
Some upper receivers will be super tight and some lowers will likewise be the same...One you get both upper and lower with very tight tolerances you will sometimes get fitment problems or you will need to make adjustments to get everything to fit correctly... I have noticed for example Aero Precision uppers come with very tight tolerances and often have to be fitted to the lower receiver as a result...this is also true with 80% completed lower receivers. The best part of that equation is you will get a more accurate fit and better accuracy etc. I will be completing some 80% lowers soon and read that some uppers require a little more cutting for a proper fit. I suppose some could benefit from a little less, too. The specs and tolerances for mil-spec are clearly spelled out, though. |
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