... you're overcharging it. Do you chill the gun before you fill it? Do you vent off gas after an initial charge to help cool it down? Don't do those things (as much). Yeah, more CO2 per fill equals more shots per fill, but that only holds true up to the point of overcharging it (which your symptoms seem to demonstrate quite well). Weighing the gun before/after filling is the most accurate method of ensuring proper fill, but I honestly don't know what the capacity of the short-barreled 115/116s is supposed to be.
If you want more shots, get a 111/112 gun (added bonus: longer barrel=better accuracy & more efficient use of CO2).