I think that by reloading for the 16 gauge, you will find that you can increase the versatility of the gun by 150%! You are right.....very limited offerings by the factories, and it's getting more and more difficult to find even the promo loads (duck & pheasant, dove & quail, ect) at the local Wally world or Kmart. Heck, I was in a Gander Mountain store the other day and they had little in the way of 16 gauge shells, and those that they did have were outrageously priced!
While there are fewer offerings in 16 gauge loaded ammo, there has been an increase in the number of different wad offerings in the past few years. One company (I think it's downrange) is suppose to come out with a WAA16 clone (great wad, IMHO) and there are a few from Ballistic products for 1 oz loadings. For a while, the only wad available was the SP16....great for field loads, but left something to be desired for target shooting.
DOn't know much about the 6mm, other than the outfitter I hunted with several years ago (in MT) had bought one for his sons, and had taken several mule deer and antelope with that gun!