I'm not sure what the Mossberg advertising department was attempting to convey with the photo, but here's a possible:
Since the lead man is in the "fatal funnel" on stairs, he going to pull the door open with the sling (not a bad idea in terms of field expedient measures, but shows lack of planning) attached to the door knob(?) and give himself the advantage of distance. Tactically, of course, both of the shotgunners should have their attention focused on the possible threat rather than their boots, and one should be positioned to the right of the door to "slice the pie" from that angle and provide cover fire for the #1.
Just guessin'
Denny
