The reason I use 2 1/2" shells is they are below the pressure limits I decided were safe for my particular gun. Since you are going to reload, you can certainly use 2 3/4" shells and load to your comfort level on the pressures. Personally if I reload for a gun with 2 1/2" chambers I might still use the longer plastic shells (thinner walls projecting into forcing cone than paper shells) but with the pressure still appropriate for the gun. You can go to a roll crimp and lower the pressure a bit too. Lower pressure doesn't necessarily mean lower velocity.