I need help determining what size chamber on a 120800 serial number gun. Or if I can fire 2 3/4 through it. Thanks

Proper measurement with appropriate guage would be required for your specific gun.
Your gun most likely came from the factory with 2 1/2 to 2 5/8" chambers.
Short shells are availabe from many suppliers. RST advertized here and I personally use their ammunition with great satisfaction.
Bob Jurewicz