If I were to consider a single shot, it would be a nice muzzleloader. You can stay at 12 ga. and load the powder charge as desired. You then have the frame for a multitude of other barrels.

I like mrbb's suggestion. The firearm's steel is the best way to drop weight. An 11-87 is a sweet gun and will reduce felt recoil because semi-auto. You have two back up shots just in case....