GLM, ALL the el cheapo 16ga 1oz loads are made to higher pressures than those for which your Flues was designed. You may well get by shooting those shells. I did with a Flues 16, as well as with a pre-WWII Sauer 16. But it's not the wisest thing to do. You can end up shortening the life of your gun.
Simply lengthening the forcing cone will do as much to lower pressure as lengthening the chamber, and you're taking out a lot less metal in the process. But even with lengthened forcing cones, I would not shoot 2 3/4" American shells in a Flues 16. If you join the 16ga reloading group, you can find out about working up 7/8 oz (some even use 3/4 oz) low pressure reloads, using standard 2 3/4" hulls. The hull length really isn't the major issue (especially if you lengthen the cones). Rather, it's the pressure the shell generates.
