assuming a 0.2g pellet, with muzzle velocity of 400fps
400fps = 120m/s, avg speed = 60m/s assuming constant acceleration
barrel time = 0.55m/60m/s = 0.0091666s
acceleration = 120m / 0.009166s = 13,091 m/s^2
force = mass * accelleration
force = 0.0002 * 13,091m/s^2 = 2.62N
barrel area = 0.0000283m^2
pressure = force/area = 92.66kN/M^2 = 13.44psig (pressure over atmospheric)
That's kind of neat because that corresponds to an atmospheric pressure of 1.91atm which is roughly in the ballpark of the 1.7 factor that venture is quoting.
Want nearly free GBB gas?