Check out my links in my post #35. (Or use the links on my profile page) I got 1.2ga from them, and if there are math errors it's my fault, but the mistakes were made by the sites.
The difference between 1.2 and 1.125 gauge is rather trivial. That new laucher is probably "backbored" a little to help keep the recoil and firing pressure in check, and I'm sure the maker actually gauged the barrel.
As to recoil, this thing is popping out a projectile that probably weighs significantly more than two pounds, so a little backboring would probably be desirable.