You can get proper pipe cutters for it macro line that leave a perfect straight cut, but i think this would be over kill for the amount of cutting you would need to do for the macro on a paintball marker.
A new blade in a stanley knife always does the trick, place it on a hard surface (that you dont mind getting scratched!) and push the blade down firmly and it cuts straight through leaving a good edge. Avoid the sawing action trying to cut it, will end up a mess and most likely cause a problem trying to get it to seal in the fitting.