The geometry of the area around the the neck of 12g carts and the piercing pin may have considerable effect on the "reservoir" effect of the entire CO2 tube. Piercing and sealing effects are all to be considered. I have no doubt that the conditions of equilibrium within and external to the cart it's self has a bearing on any scheme to control gas flow. At this moment, thoughts run through wondering how the addition of a sealing device (o-ring perhaps) on the strike pin that would isolate the valve body volume during shot cycle might offer a degree of control to the total gas charge available.
Bulk fill condition may be more easily contemplated.
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I am sure others have gone before on such mumblings, but I have not come across their notes.