The power band of a precharged airgun
by Tom Gaylord
Writing as B.B. Pelletier
This report covers:
- Difference between a precharged airgun and a multi-pump
- Balanced valve
- Benjamin 700
- Benjamin 710
- Benjamin 600 Automatic
- How a PCP valve works
- Run out of barrel
- Power band = 1000 psi?
- The power curve
- Why don’t they…?
- What should you do?
- Last tip — worth the price
The topic for today’s report comes from reader GunFun 1, who asked me to discuss the useful power band of a precharged pneumatic (PCP). Some of you are thinking about getting into PCPs and you wonder how they work. Today’s report should clarify some of that for you.
Difference between a precharged airgun and a multi-pump
I’ll start with the main difference between a multi-pump and a PCP. A multi-pump like the Benjamin 392 that many of you are familiar with, fills with air that you pump in manually. It all gets exhausted with the one shot, or at least it is supposed to. Fill with many pumps and then exhaust all at once — that’s something many airgunners know.
Sometimes the valve in a multi-pump can’t exhaust all the air. The striker spring weakened over time. When that happens, a second shot can be fired without pumping the gun. That shot is usually very weak, although as the spring degrades, it can become more and more powerful, until it has some real power of its own! What is happening is the valve is not exhausting all the air on the first shot and some of it remains in the gun for a second shot. This is what happens with a precharged gun, only it is planned and the second shot is very close in power to the first. And so is shot number three and so on.
It would be possible to create a multi-pump that fires a second shot powerfully. All that’s needed is a striker spring that’s weak enough, a valve return spring with the correct tension and a valve orifice and valve seat shape that exhausts air in smaller amounts. We already know that it happens to multi-pumps though wear, but it can also be planned and designed into the airgun.
The Benjamin 700 from 1937 was a 25-shot BB gun that was pumped 12 times, then got about four powerful shots before needing to be topped off with a few more pump strokes. It was advertised that way, so Benjamin meant for it to be used that way. They were apparently fascinated with the performance, and continued to advance the design in the model 710.
The Benjamin 710 of 1939 is another 25-shot BB gun, only this one gets pumped up to 20 times and gets 6 to 8 powerful shots on one fill. This one they advertised could maintain full power with one or two pumps after each shot. This was the last model of the 700-series that got multiple shots from a single fill of air. The model 720 was also a 25-shot BB gun, but it had to be pumped after every shot.
Benjamin 600 Automatic
This BB gun is a semiautomatic that keeps firing as fast as the trigger is pulled. Fill it with as much air as you can pump. It’s a front-pumper, so it won’t be that many strokes. Benjamin’s instructions say to fill it as much as you can, which they say depends on what you weigh. From this you should get 10-12 shots before refilling.
I could continue, but you get the idea. That fact is Benjamin made several pneumatics that got many shots per fill. All you have to do to make a gun like this into a PCP is take away the onboard hand pump and fill the gun from a separate source.
How a PCP valve works
The information above illustrates how a powerplant and its firing valve can be set up to release just a portion of the air stored in the reservoir, keeping the rest for follow-on shots. All it takes is some thought to enlarge the reservoir to get even more shots per fill and to adjust the valve to use that compressed air as sparingly as possible. But something else creeps into the equation at this point.
With the Benjamin multi-pumps described above, each successive shot was less powerful until all the air was exhausted. But PCPs don’t work that way. A PCP will continue to shoot all its shots within a reasonably tight velocity spread, even though the air pressure inside the reservoir is constantly dropping. They can do that because when the valve releases air, it stays behind the pellet and holds the valve open for a brief time. If the barrel is long enough, the pellet stays inside for some time and allows the air pressure behind it to continue to build because the valve has not yet closed.
If the valve did close, then each shot would be a little slower than the last. But because the air valve remains open just a little longer each time, more air leaves the reservoir. Yes it is pressurized a little less each time, but there is more of it because the valve remains open just a little longer and that balances things out. As a result, the pellet gets a longer push from lower pressure air, which keeps the pellet velocity more or less the same.
Run out of barrel
At some point, though, the pellet has to leave the muzzle and the push has to stop. When that happens, the pellet velocity starts dropping with every successive shot and we say the gun is off the power curve.
Power band = 1000 psi?
GunFun 1 mentioned that he has noticed that many PCPs seem to have their power band — the pressure at which all the shots seem to go out at about the same velocity — in a range of pressures that’s about 1,000 psi, high to low. But he then said that his Talon SS is only filled to 2700 psi and runs out of steam at 2000 psi, which is only 700 psi. The lesson here is that 1000 psi is not a magic number, any more than a fill pressure of 3000 psi is magic. And please note that the Talon SS has a 12-inch barrel, although I don’t know if GunFun1 has replaced his barrel or not. I’m just saying that a shorter barrel will have a narrower power band.
When I worked at AirForce Airguns I used to take angry calls from customers wanting to know why their AirForce Condor could only be filled to 2650 psi, when it “should” have filled to 3000 psi. They tried putting in more air, but their gun just shot slower when they did. They were still getting the top velocity the gun could give (1,250 f.p.s. with .22 caliber Crosman Premiers) and they were getting it over 20 shots, which is all a Condor will do at top velocity, but they wondered why they were being “cheated” by having their gun stop 350 psi below the place where it “should” stop.
The power curve
These shooters did not understand how precharged airguns work. You don’t use an arbitrary fill pressure to determine the gun’s power. You use a chronograph. You fill the gun until any higher pressure results in slower shots. That pressure is the maximum fill pressure — regardless of what number that turns out to be. Then you shoot the gun until the velocity starts dropping off rapidly. Fill it again and note the pressure at which the tank begins to accept air. That is your lower pressure limit.
The band of pressure between the highest fill pressure and the lowest pressure at which the shots are still stable is called the power curve. Don’t try to make that curve conform to certain numbers. Just find out where it is and live with it. The high number is your maximum fill pressure and the low number is the point where your gun needs to be filled.
As it turns out, many times the power curve for a particular airgun will be about 1,000 psi., but it doesn’t have to be. It is whatever it is, and you find that out with a chronograph. Typically PCPs with shorter barrels will have narrower power bands, as will the more powerful guns. A lower-powered PCP and one with a longer barrel will have a larger band of pressure. But, until you use a chronograph to find out, you’ll never know for sure.
Why don’t they…?
And this is where the armchair engineers pipe in with all their suggestions for building better airguns. If only the engineers that designed PCPs would make them with broader power bands so we would get more shots from a fill. That’s so obvious that they wonder why no one has thought of it before.
Well, relax, because “they” have thought of it. In fact, “they” are constantly working with things like air transfer port diameters and length, striker weights and strokes and valve return spring strengths and striker de-bounce devices and so on. “They” have even resorted to electronically operated valves that are computer controlled. “They” have given us the very best “they” can, thus far, and “they” will continue to work on improving things in the future. But until then, power bands will probably not exceed 1,000 psi and many times they will be narrower.
What should you do?
Now that you know this, what should you do about it? Glad you asked. I’m assuming you want to apply it to a precharged airgun. First, get a chronograph. You don’t need a chronograph to enjoy a precharged airgun, but you will never know exactly where the power band is without one.
Use the published fill pressure as a starting point — a guideline. I have seen plenty precharged airguns without a regulator whose maximum fill pressures were not what was published or even what was engraved on the side of the gun. If it says 3000 psi, expect something between 2800 and 3100 psi. That’s why you test every airgun.
I have found that the lower the fill pressure the closer the number usually comes to the actual maximum. I have owned two different Daystates that filled to 2600 psi, and they both tested to within 100 psi of that. A hand-built gun like a USFT is probably tested by the builder before you get it. If a gun like that is off, it will be because your gauge doesn’t agree with the maker’s.
The point is, you only discover the power band of a PCP by testing for it. The test is simple if you have a chronograph. Without it, you are just guessing.
Last tip — worth the price
My last tip today. What if you don’t have a chronograph and can’t afford one? Is there any way to know what your power band is? Yes, there is.
You may not know the velocity the gun is delivering, but you can discover where the power band is by shooting a large group of shots at a target long distance away. First, fill to where you think the maximum is. Then start shooting at a target a long way off — at least 40 yards. If the pellets walk up in the beginning, the gun was over-pressurized. If they group together and then start to drop, the gun was on the power band until the pellets started to drop. That’s when it came off the power band. I actually did this with my first Career 707 rifle.
Just remember, you don’t need a lot of extra equipment to enjoy an airgun. But if you want to understand the technical parameters, some equipment is vital.