Your Francotte should be fine with most 2-1/2 dram 2-3/4" field loads. I like the Winchester Super-X Australian shells, and the Remington Game Loads. Use them in my Parker and LC Smith 16's with no problems. The Parker is 1906 short chamber, and the Smith is 1928-29 on a Featherweight frame. The Federal field loads seem a good bit stouter than the others, so I save them for the Model 12's. You can get short 16's from ARMUSA, RST, Kent Gamebore, among others. Pressure is more important than shell length in the old 16's. Don't lengthen the chambers, but consider easing the forcing cones.
The only reason to use Gauge Mates in an old 16 is so that you can bum 20 gauge shells from your buddies.
