I'm sure the good Rev has the information somewhere in his post above, but, there is NO reason you can't use 2 3/4" ammunition in a 2 1/2" gun-with one BIG caveat-the 2 3/4" ammunition has to be loaded to no higher level of pressure than the 2 1/2 inch ammunition the gun was designed for.
Assuming the gun was proofed in France with powder T (most guns since 1900 or so) that is a pretty high level of proof. Most of those French guns have pretty heavy wall thickness, but, they are light guns. The pounding YOU will take with off the shelf ammunition, won't be all that pleasant, howver.
The gun is a bargain, compared to what some dealers seem to think 311s are worth these days, so take some of the coin you saved and buy a case of the low pressure stuff from Eley or RST, or Polywad, or W. Richards, or whoever.
Sounds like a great deal of fun to me.
Best,
Ted
