Quoted:
Bushmaster currently does not replace or rebuild preban receivers and then reissue them with the same serial number.
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Actually, that's untrue. But they won't publicly admit to doing it, even though they have done so in the past, because they don't want lots of people requesting replacements for cosmetic reasons.
Due to Bushmaster's manufacturing structure, they tool up for making lowers, and then make a whole bunch of them. Then, they retool and make other items. Because the lowers are serial numbered during manufacture, they can't make a replacement lower with an out-of-sequence serial number until the NEXT time they make lowers. Which can be a year or more. Plus, it's a PITA to deal with.
But, again, they HAVE done replacement "repairs" for catastrophically-damaged pre-ban lowers before, and I'm sure would do so again. But they won't do it just because you ask them to; generally, if they can't repair it, THEY will initiate the replacment.
-Troy