The sniper II pumps and autocockers have a hammer that hits a poppet and releases the compressed air or CO2. It fires just fine when using any direct air source as long as that air source is consistent. An inline regulator just guarantees that the pressure is always constant and makes the shot to shot consistency better. Without the regulated input air, the velocity will go down as you deplete an air tank or as CO2 pressures fluctuate with temperature. If the temperature stays consistent during the day, CO2 pressure also stays very consistent and predictable.
Except for the Automag in front, its usually the man behind the equipment that counts.