i took a dremel and sanded the inside down and now my hoppers fit in there tight and work great
2nd) why are you using co2 on it, you need to use HPA.. i use HPA on mine and it shoots like a beast. i can outshoot my sons proto in psp ramp mode and my mg is on semi auto, pull one shoots one.. you can put it into full auto by putting the jumper on the board but theres no need to and most fields done allow this.
as far as your tank only going to 100 stop using co2 cause its going to mess your gun up. one possibility is the pin may not be depressing the tank all the way to allow the air to come out.