I'm sure this is fairly obvious to a lot of you guys, but the reason you see the POI being fairly consistent throughout the cartridge, then dropping rapidly right near the end is due to the nature of CO2 and how it behaves at room temperatures. CO2 has the interesting property (shared with propane and a few other gasses) of being liquid at a reasonable pressure at room temperature, and we can therefore use a phase-change to help supply more shots from a cartridge. How many shots do you think we could get out of a 3000psi compressed air tank the size of a CO2 cartridge? I have no idea, but it starts around "not many" and goes from there!A 12-gram cartridge is full of liquefied CO2, which has a natural pressure at room temperature of around 800psi. That is to say, the liquid at room temperature requires 800psi of pressure to keep from boiling. Since the cartridge is very nearly full of liquid with just a bit of gas over top, as soon as you valve off a bit of the gas (say, firing a shot), the pressure drops very slightly, and the liquid can boil. It does so very briefly, until the natural pressure is re-established, and the boiling stops due to pressure, right back at ~800psi. So, your reservoir pressure will remain very steady at around 800 and your POI the same until all the liquid is gone. From THAT point, it behaves just like a PCP with 800lbs in the tank, and each shot sees a decreasing pressure behind it and your POI starts to drop between shots. Your POI will be different in summer than in winter, since the cooler temperatures reduce the boiling pressure of the CO2, and the natural pressure will be somewhat lower. This can also be caused by rapid fire, since boiling the liquid CO2 to re-establish pressure requires thermal energy, which is pulled from the surroundings. As succeeding shots drop the temperature of the metal around the cartridge and reservoir, there is less available energy to boil off the next shot's gas charge and the overall pressure drops slightly. In slow fire, the material can re-build enough thermal energy from the world around it to keep things stable, and your POI the same.