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I've assembled more than a dozen AR lowers and 1/2 dozen uppers. I've found numerous combinations of parts that didn't fit together in a way that satisfied me. I also found several combinations of upper to lower that were just a no-go. With several in the rack, it's usually fairly easy to get a good fit by trial and error. Most of my lowers came in lots of 3 to 5 and it seemed each lot(even of the same brand) was slightly different as far as pin holes, parts fitment, and deck height. It's all about tolerances and whether they stack or cancel each other out.
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