Technician1002 wrote:Low pressure or high pressure, the distance the projectile moves as the valve opens scales to the rate the valve opens, so high or low pressure, the mass ratio does play an important role in how far the projectile moves before the valve is fully open.
Considering the piston travel required to obtain maximum valve flow (Varies from about 1/4 to 3/4 of the barrel port diameter, depending on the configuration of the valve), the mass ratio isn't too important.
For example, the 0.7" porting T valve on my high pressure copper pneumatic requires a piston travel of ~0.5" for maximum flow. Even if you assume the forces acting upon the projectile and piston are of the same magnitude (Which is not the case, as the piston is larger in diameter), and ignore the numerous other factors, a piston with 10 times the mass of the projectile could be used, and the valve would still fully open before the round had traveled 1/4 of the length of its relatively short 20" barrel.
Obviously the effect will be more substantial with longer piston travel distances, but the use of a more realistic piston mass brings it back down to the insignificant range.
Also, I see your gumball pierced soda can, and raise you one paintball pierced soda can, done with a medium length 4ft barrel, and a M<sub>piston</sub>:M<sub>projectile</sub> ratio of 7:1.