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Why would you?
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Most of the reasons are related, directly or indirectly, to the KSG having been designed as a bullpup from the ground up vs. the BU 870 being a conversion/drop in kit.
But, beyond that broad consideration, IMHO the KSG wins in terms of:
1) ease of loading/operation (slightly less awkward)
2) slide release (the BU interface in this regard can be problematic)
3) trigger (much better)
4) materials/durability. (my BUs are both 1st gen; I understand newer ones are more robust but haven't tried one)
5) aftermarket accessories/support (plenty of stuff for a std. 870, but once in the BU chassis most is no longer applicable)
6) capacity (of course)
FTR, I am a huge 870 fan, own more than a dozen including 3 SBSs and an AOW... but in this particular narrow application, I would opt for the Kushnapup first (I have 4 S12s, but only one in bullpup trim), KSG second, and BU 870 third, Mossberg bullpup never, no experience with the UTS whatsoever.