Physics of CO2 Gun: Myths & Reality
Looking through the web sites, forums and blogs devoted to Air Guns we see that users have a lot of questions concerning CO2 guns functioning. And we also notice that there is a lot of information that is not true. For example a person who was interested in Drozd CO2 BB pistol asks an air gun store manager if the muzzle velocity will decrease with every shot. A seller answers that it definitely will, because with every shot the amount of CO2 remaining in the cartridge became less. When I hear statements like this I want to become a writer (at least for a while) and to explain other people what really happens.
So, why the muzzle velocity will not drop?
CO2 guns and pistols are powered by CO2 (carbon dioxide). Notwithstanding a cartridge kind, volume and manufacturer it carries liquefied CO2. The liquid fraction constitutes 80% of CO2 gas in a new sealed cartridge (unless the cylinder was underfilled). 20% of volume is a gas fraction.
Pressure of gaseous CO2 depends on the temperature inside a cylinder (that is usually equal to ambient air temperature). Energy of this pressurized gas is used to power CO2 pistols and rifles. When a portion of gas is spent for a shot a portion of liquefied CO2 turns into equal amount of gas immediately. And interior pressure in a cylinder remains the same. And this process keeping pressure at the same level for quite a long time continues till there is no more liquefied fraction of CO2 in a cartridge. Before this point pressure of gas can be considered as constant. This effect of pressure self-stabilization allows using CO2 to power Air Rifles and Pistols including ones designed for shooting sport.
When all liquefied CO2 is spent some gas will remain in a cylinder and its volume will be equal to volume of a cartridge. After reaching this point the pressure inside the reservoir drops very quickly. It is recommended to waste the rest of gas shooting without pellets before removing a cylinder. If you continue to shoot with pellets there is a risk of pellets jamming inside a barrel because pressure becomes so low that it is not enough to let a projectile leave a barrel. If you do not dry-fire a pistol or a rifle before removing a cylinder a low temperature of CO2 gas escaping from a cartridge when you are taking it away will damage rubber seal. So you will have to change rubber o-rings in order to prevent leakage.
Now let’s try to think how many shots per one cylinder we will have. At atmospheric pressure one liter of CO2 weighs 1.98 gr. So we will get about 6 liters out of 12 gr. reservoir. According to Boyle-Mariotte law for a given mass of gas multyplicating volume by pressure we will get a constant figure (providing the temperature does not change).
P1 x V1=p2 x V2=…const (t=const, m=const)
So at room temperature and 60 bar pressure we will get 99 cc of CO2. Now we deduct 15 cc (a volume a cylinder), that will be spent for the last 10-20 shots without pellets (that we have already mentioned).
This 84 cc will give an energy for full-power shots, that we will make till some liquefied gas remains in a cylinder.
Now, how much gas is needed for one shot? Barrel pressure (a pressure that force a projectile to leave a barrel) is 20-40% of maximum pressure. Knowing a barrel length we can calculate its volume, for example a volume of 6’’ barrel is about 1.5 cc. So, for one cartridge we will have
84cc : 1.5cc=56 shots.
Using Boyle-Mariotte law we can calculate that at 32oF and 30 bar pressure we will have 264 cc of CO2 and twice more shots but their power will be twice less. Under these conditions it will be useless to change a cartridge after a couple of clips, it will not make shots more powerful.
But still when we shoot very quickly we notice that velocity drops. Why does it happen? The matter is that a process of turning liquid fraction into gaseous one demands some heat energy (so-called heat-absorbing process). When you shoot very quickly a temperature inside a cylinder drops, heat energy is not enough and the process of turning liquid into gas slows down.
Thus pressure of every new portion of gas will be lower than pressure of a previous one. In order to prevent this effect it is necessary to make intervals between shots enough to restore a temperature inside a cylinder till air temperature level. Some authors say that when you shoot quickly a pressure inside a cylinder decreases because there isn’t enough time to turn liquefied fraction into gaseous one. But this is not right.
Muzzle velocity depends on gas pressure, volume of gas portion and distance at which a power is affecting a projectile (barrel length). So the fastest velocity will have a gun with longer barrel providing temperature being equal and a portion of gas is optimal for this gun.
Optimal portion of gas allows achieving a high velocity while a volume of spent gas being acceptable. There is also a maximum portion of gas that moves a pellet with a maximum velocity for a given barrel. For example if optimal portion of CO2 will force a projectile to leave a barrel at 360 fps and we will have 40-45 shots per one cartridge, with maximum portion of gas a muzzle velocity will be 380-385 fps and a cartridge will be enough for 25-28 shots. Further increasing of gas portion will give nothing. At the same time if we will take longer barrel, for example 8” instead of 6” a velocity will be 50-65 fps more (gas portion being the same).
It is impossible to achieve such a result by increasing a portion of gas.