In my searching I also found some 10 round drum and 10 round box mags for Mossberg 88's/500's and 590's. Both require modification but are not that expensive.Yeah, I've had two of these combo-packs in my time. 12 gauge and 20. Each with one long sporting barrel and one shorter slugs / buckshot/ home defense barrel.
I don't think a dedicated "combat only" shotgun is worth it for me. I've got handguns and carbines for battle. I want my shotgun to primarily be sporting, for busting clays or hunting game. But for $79 extra, it's not much of an investment to get the short barrel to go along with the long 26" or 28" sporting barrel.
And for the 12-gauge guns, you can get a magazine extender tube that will let your gun hold 9 or 8 rounds, instead of the usual 5 or 6.