Chamber length wasn't marked on all French guns, but I'd be skeptical of any that are 70MM/2 3/4" and not marked. May well have started life as 2 1/2". One ounce loads in a 5 3/4# gun can give you a fair jolt. If you're going to shoot factory loads, you might consider RST's 7/8 oz shells. They even make 3/4 oz loads for the 16. Pressure isn't really an issue with recoil. Reduce either shot charge or velocity, or both, and she'll kick less.