Because modern plastic cases are much thinner than the old paper cases that this gun was made to use, the pressure increase with 2 3/4 inch ammo in the short chambers, while there, isn't a huge increase. One big problem here is determining the actual chamber pressures created by the ammo you intend to use. The common assumption is that target loads, especially "light" loads are also low pressure. Not so, many target loads are in the upper 20% of acceptable pressure. Not a problem in modern guns, breaks targets, burns clean, what's not to like? Most manufacturers don't advertise the pressures of specific loads but may provide them on request. Another option is reloads made to exact specifications with published pressures, look for pressures less than 8,000 psi. No fudging, every component must be exactly to recipe. I would still strongly suggest that this gun should be professionally checked out. Using higher pressure ammo than the gun is made for may not blow it up but at the least it will strain this old gun and loosen it up prematurely. I thought CAS used black powder??
