..are only a few guns where you can bulk fill or use 12gr. WITHOUT doing valve mods. Using a tanker valve in a 22Xx won't do for a direct comparison. The new valve would change flow patterns, return spring tension, etc. Needs to be the SAME valve.
QBs can be bulked fill with NO changes from the 12gr. setting...they get more speed. Can't be becasue the pressure is higher as no matter how much co2 is on tap, if the temperature is the same, the pressure is the same as when suing 12gr. Can't be better gas flow, as it is the exact same valve. So why do they go faster?
Do have an old Crosman 2000 12gr. rifle. Made for really low speeds (like middle 400's). But with a chunk of cheap flashlight machined for two o-rings to bridge the vent hole (not unlike how Discoveries mount a gauge) and a bulk fill end cap, can bulk fill them with the exdact same valve in place. They also show an increase in speed.
So I'm guessing, if everything is kept the same from the valve to the muzzle, the differnce is that the piercing pin is using the 12gr. as a mechanical stop...the valve just can't open any farther than when the piercing pin runs into the 12gr.