I do not believe using 2 3/4 shells in a 2 1/2 chamber is wise. It may not blow up the gun so could perhaps be called safe, but it has to raise pressure as the shot and wad column are restricted for a very short span of time. Without a means of taking pressure readings one can't know how much it is raising pressure. I use what the gun is chambered for.

NRA Benefactor 2008
NRA Patron 2007
NRA Endowment 1996
NRA Life 1988