I wasn't sure if this was an engineering question or a physics question, but decided to pose it here in the general engineering section. A friend of mine, Blake, has a high-end air rifle that exhibits some puzzling behavior. The highest velocity for the pellet doesn't occur when the internal air tank is at its highest pressure, but instead occurs after the pressure has dropped a fair amount. The Excel chart below shows a bunch of data that Blake has captured, with the velocity, in fps, along the vertical axis, and the internal tank pressure, in bars (1 bar = 14.7 lb/in2, if memory serves) along the horizontal axis. We're most interested in the lowest graph, in black. In this graph, the peak velocity is about 900 fps, when the tank pressure has dropped from 190 bars to 120 bars (from about 2800 psi to about 1800 psi). Obviouly, this is not a Daisy BB gun. The internal tank can be filled using a foot pump, but Blake uses a scuba tank to fill the tank on the rifle. When Blake asked me why this might be happening, my first thought was that at the higher pressures, some of the pressure might be escaping past O-rings, resulting in less pressure to fire the pellet. He replied that if you start off at the lower pressure, you still get the same sort of concave-down graph, so my theory went down in flames. My next and final thought was that the peak came after 25 to 30 pellets had been fired, so possibly the friction of the pellets passing through the rifled barrel was causing it to heat up and shrink to make a tighter fit for the pellet, with less air passing around the pellet, and more air propelling the pellet out the barrel. Does anyone have any ideas about what could be causing the max velocity to peak after the tank pressure has decreased (and after several pellets have been fired)? I don't have any other ideas to offer. Notes: 1. The velocities are measured with a photo chronograph -- I think that's what it's called. 2. The four graphs come from tests different numbers of turns of the linkage between the trigger and a gizmo that releases air from the internal tank to the chamber behind the pellet. More turns increase (I think) the amount of air that is vented. 3.