There is a vast difference in an "All Purpose Gun" which does not exist & a "Multi Purpose Gun" which does. A 12 or 16 ga weighing 6 3/4-7 lbs & capable of handling shot charges up through 1¼ oz (low velocity for recoil) Will handle an amazing variety of game.
"All purpose" is an accurate description. Perhaps a list of the purposes needs to be explored. I completely agree that every benefit a particular gun has results in a liability somewhere else down the line...and no all purpose gun would be the best gun available in more than one catagory.
A mechanically sound 12 gauge with choke tubes will take, effectively, any north american small or medium game...including black bears. I wouldn't want to pursue moose, antelope, brown bear, or elk with one, although I know it could be done. The same gun will function on the sporting clays course, trap range, or skeet range... well enough to beat the people I would normally beat.
I'd even go so far as to suggest that an I.C. or Modified fixed choke would do just fine on any of the game listed.
