I'm not sure about the Mossberg, but the 870 will not break down as short as an over/under unless you remove the magazine tube, which kind of takes them out of the "break down" category. A Winchester Model 12 will, if it has a short barrel, but that is not what we are talking about here. I love my 870 with two barrels also, but I can see a place for this little o/u...
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OBG