Matt,

Pistol is out, right? (NYC).

Neither the AR-7 nor the M6 are good substitutes for a 12 gauge pump for self-defense. They have their uses, though.

What about a pistol-caliber short barreled (16") lever action carbine? Although... wall over-penetration will be an issue UNLESS "magic bullets" like Glasser safety bullets are the "urban load". (If you choose this route, I would go with either 44 Rem Magnum or 45 Colt as the caliber - they are far more versitile, especially the 44 if you are not a reloader).

Another alternative would be a short and relatively light 12 gauge single shot for kayaking - keep your 870 for the apartment.

I'm compelled to note again - despite what someone recently posted - on at least the first interior wall, it does not matter what size shot - a close range "miss" into an interior wall will completely penetrate two layers of wallboard (some old-fashioned plaster walls CAN be more resistant to over penetration by a shot charge - construction details matter greatly).

Anyway, a 410 with any load is not a very effective stopper - terrible wound, yes. I think you're asking to compare apples and oranges. If I wanted a combo gun, I would prefer a 22lr / 20 gauge (gee, I think I have one of those...).

YMMV...

Regards,

Tom