I walked out of my local gunshop today with a very nice used 870. It's got a 20" on it now, came with a few chokes. The barrel is marked for 2-3/4 shells only. I was wondering could I buy another barrel for 2-3/4 and 3" shells or can I only stick another 2-3/4 barrel on it? And if I got a 3" barrel, could I fire 3" loads in my shotgun?
The only real difference between a 2 3/4" gun and a 3" gun is the barrel and the ejector assembly.
If you have a gun that was originally made as a 2 3/4", you can put a 3" chambered barrel on it, and fire 3" shells.
BUT, the gun won't eject the longer 3" shell.
You can have your gun converted to handle 3" shells by having a gunsmith install a longer 3" ejector assembly in the receiver.
This isn't an expensive or particularly difficult job, BUT it requires some special tools to do it, so it isn't a job you can do at home.