16 gauge easier to find than the 20? I've never seen that anywhere. I suppose it depends on your location. Are you in France perhaps?
The reason I get involved in these discussions is the nonsense propogated that there is something magical about the 16 gauge. There isn't. Regardless of what you are shooting if you put the pellets in the right place gauge doesn't matter. The 16 ounce to a pound, 1 ounce payload equals a square load and perfect pattern doesn't make sense either. Read Brister. Of all the qualities that affect pattern shot hardness and velocity are the most significant.
If you like it great. Use it, enjoy it and kill as many birds as you can. But don't try and make the gauge into something it isn't.
The advantages of a 16 gauge frame with 12 gauge barrels are twofold. The first is weight. It's lighter. Second is flexibility. I load 12 and have ready access to a good assortment of factory loads. I also don't have to listen to the clerk at the gunstore sneeze when he blows the dust off the box of 16 gauge that has sat there since the early 70's.
I have no idea what you are getting at with your comment about extra long firing pins. As for the price that's between me and my banker.
Overall don't take my comments too much to heart. The most fun I have with the 16 gauge is rousing its more excitable fans to spirited Internet debates.