Well whaddya know... learn somethin' new every day.So can the 870. Ever noticed the step-down on the front half-inch or so of factory Remington extended mag tubes? That step happens to be the same diameter as the M16 flash hider. There is a barrel band support bracket available that latches the bayonet in place.
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I bought an 870 with a factory installed +2 mag extension and it was a no go! The magazine spring would get caught inside the magazine tube and the shells would flop around. The mossberg is good to go! Just my $0.02