How does a pneumatic airgun work?
by B.B. Pelletier
I got a request for this article last week from Manish in India and then another request to explain what is meant by the term valve lock, so I'll deal with both subjects today.
There's very little difference between the valve in a multi-pump pneumatic and a precharged pneumatic. Add air volume to a multi-pump reservoir and you can make the gun capable of multiple shots on a single fill of air. Then the only difference between that and a precharged gun would be the presence of the onboard pump. Therefore, the valve I will discuss applies equally to both types of pneumatics.
First, let's look at a closed valve (graphic below). The reservoir is filled with compressed air that wants to get out, but the valve is blocking its way. Air pressure on the valve (red) presses it against the valve seat, which is a part of the valve body (black). The walls of the reservoir tube are also shown in black. A valve return spring inside the valve body also holds the valve shut, but with far less pressure than the air itself. The purpose of the return spring will be obvious in a moment.
When the gun fires, a weight called a striker or hammer is driven forward by spring pressure. It strikes the end of the valve stem (blue), driving it forward. When it goes, it takes the valve with it, opening small air passages around the valve stem that allow the pressurized air to escape.
What is "valve lock"?
Because the air in the reservoir exerts force on the valve, holding it closed, the higher the pressure, the more force it exerts. That force is balanced against the size of the valve, the weight of the striker and the strength of the striker spring. There's a range of reservoir pressure in which the valve remains open long enough to pass the same amount of air, despite the fact that the air pressure inside the reservoir declines with every shot. When the air pressure is at the high end of the range, the valve remains open for a shorter time, but the higher pressure forces more air through the valve. When the air pressure drops, the valve remains open longer, allowing a greater length of time for air at lower pressure to flow through the valve. The same volume of air flows through the valve in both circumstances.
The result for a precharged pneumatic that gets many shots per charge is that even though the air pressure is dropping, the velocity remains more or less constant throughout this pressure range. For a multi-pump pneumatic, the pressure range doesn't mean as much, except when the pressure goes too high. Then the valve cannot remain open long enough to exhaust all the air in the gun and there will still be air remaining for a second shot. Also, the shot the gun fires will be slower than normal, because less air is behind it.
The velocity will drop for both precharged pneumatics and multi-pumps when the air pressure goes above the range for which the valve was designed. The reason should be obvious - the valve closes before all the air needed for the shot can get out. This marks the beginning of valve lock. If the air pressure goes too high, the combined force of the striker and its spring will not be able to open the valve at all and there will be no shot. Now the gun is locked down by its own air pressure. The only remedy is to remove some of the pressure to drop the internal pressure down to a level at which the valve can open. Continued firing will drop the air pressure back down into the optimum operating range.
The performance curve
For this reason, using a chronograph with a precharged pneumatic allows the shooter to discover the exact pressure at which the valve operates best. And this decision can be left up to the shooter. If he wants more shots, he accepts a lower initial velocity (that comes from a higher initial pressure) and allows the rifle to climb in velocity, then fall back down to his established parameter. If he wants his shots to be as close in velocity as possible, he starts at a lower air pressure that gives him an initial velocity higher and closer to the highest velocity of which the rifle is capable. This is called the performance curve, and every non-regulated pneumatic rifle and pistol has it.
Some airguns, most notably those of Korean manufacture, are designed with sheer power in mind. Their valves are set to open at very high pressures, very close to valve lock. These guns will lock up if over-pressurized just a little, and they tend to have a steadily declining velocity instead of the curve described above. They can be modified to give longer strings of consistent shots, but the velocity at which they do it has to drop. So, a .22-caliber Career 707 might give the first five shots with .22-caliber Crosman Premiers that look like this:
If the rifle were tuned for more shots, the same first five Premiers might look like this:
This is what makes the AirForce Condor so remarkable. Not only does it deliver smashing power in the 60+ foot-pound range - it does so for the first 10 shots. And, if you'll accept a velocity loss of 75 f.p.s., the first 20 shots are usable. No other smallbore air rifle has that kind of performance curve.
The Benjamin Discovery, which needs only 2,000 psi air to get identical performance to other rifles shooting with 3,000 psi, teaches us that air pressure, alone, doesn't make velocity. The timing of the valve has far more to do with it than what's inside the reservoir.
Those are the basics of pneumatic valves. They govern the gun's performance, and they dictate the parameters under which the air pressure must be managed.