If you really want some nice Tactical shotguns...look at the Benelli lineup of guns...way better options than anything Mossberg put outs in my view:
M4 is the Benelli high end option...
The M-4 is a gas operated weapon.
They have some tactical versions of the M-2 as well:
The M-2's are all Inertia Operated
list prices are $ 1,250 - $ 2,700 or so / the M-4 would be my personal choice, if I wanted a dedicated Tactical Shotgun, and they're selling new in my area for around $ 1,850.