I built and sold a few of these.This is my personal one.
"Coming off the reg" is just a phrase that means the pressure in the reservoir is no longer high enough to maintain a consistent velocity.If the hammer strike is adjusted correctly, the velocity with gradually taper down as the pressure in the bottle falls below the regulator's setpoint. The pressure at which the velocity falls by a distinguishable amount will actually be somewhat lower than the regulator's setpoint. For example, you might have a regulator set for 1300psi and continue to get consistent shots until the pressure falls below 1100psi.However if there's too little hammer strike, you'll instead see the velocity bump up as the pressure falls below the setpoint, and then it will eventually fall back down if you continue to shoot the pressure down.