Rob: please see this thread
http://www.16ga.com/forum/viewtopic.php?t=1983The Winter 2001 Double Gun Journal has the article by Sherman Bell called "Finding Out for Myself" Part V "Long Shells in Short Chambers". After extensive testing carried out in a very professional manner he concludes:
"With loads that are sensible in a light 2 1/2 inch gun, we see no dangerous pressure levels produced. I see no reason, related to safety, to modify an original 2 1/2 inch chambered gun to shoot 2 3/4 shells,
If The 2 3/4 Inch Load You Intend To Use Would Develop Pressure That Is Safe In That Gun, When Fired In A Standard Chamber!"I would observe that shooting 2 3/4" shells in 2 1/2" chambers
might increase
recoil somewhat if the case mouth enters the forcing cone. I am extremely recoil sensitive, and feel more recoil shooting 2 1/2" shells in my 2 9/16" chambered 16g than the same shells in my 16g in which the chambers and forcing cones have been lengthened.