EDIT: Duh!  Just re-read your post and only now noticed the "double-barreled"  part.  None listed below are double barreled.

Greg Crowfoot still has his freebie Mosberg pump up at Greylight

http://www.greylight.com/poserweapons.php

Anthony Appleyard  converted it to a poseable pistol-grip version of that one which is still in FS here titled MosberMariner rustproof marine pumpaction shotgun..

, and the best freebee I've seen was by mertext (who is a vendor here), but I don't see any of the weapons collection he previously  posted anywhere here, or on his site  (it was a very nice looking Benelli Mt Tactical shotgun.)

and  last that I've dl'd, another pistol grip version from amazing.3d.