by Tom Gaylord, a.k.a. B.B. Pelletier

Today’s report came from a question asked by blog reader GunFun1:

“How do airgun manufacturers come up with the maximum fill pressure for their precharged airguns?”

He went on to say:

“I guess it is safety related and to protect the o-rings also. And if the gun isn’t adjustable for the striker and spring I guess they test to see if the gun will get valve lock or something? Or is for the fact that most hand pumps won’t go too much above the 3000 psi fill level with the exception of a few brands?”

The report
What I am about to tell you has been collected from dozens of meetings and tours through airgun companies. I’ve been in the engineering departments, talked to the development engineers, and seen the CAD systems and the prototypes they work on. I’ve also had long discussions with smaller airgun builders like Dennis Quackenbush and Gary Barnes. I know how they approach this problem. So, I can tell you what I’ve seen. That’s not the same as a full disclosure report, but I don’t think you’re going to get that anywhere.

Doing what works is Job One
Airgun companies don’t “come up with” the maximum fill pressure at all. At least they don’t do it as an initial calculated engineering step. It’s not a primary goal, except in a few instances I’ll mention. Instead, what they try to do is design and then build a precharged airgun that works. One that shoots pellets out the muzzle. That’s always the first objective.

In coming up with “what works,” each company has something definite in mind. One company may want to build a 10-meter target rifle that fires an 8-grain .177 pellet at 575 f.p.s. for at least 120 shots from a fill, and they don’t want the velocity to vary by as much as 10 feet per second throughout the entire shot string. There are several paths available to them — things like pressure regulators, balanced valves and so on.

Another company wants their gun to propel a 30-grain .22-caliber pellet at 1,000 f.p.s. They are making a hunting rifle, and their first goal is to get the power they are seeking. After they get it, they’ll then look into getting the most number of shots from a single fill.

A third company wants to build a rifle that gets a lot of shots in the 25 foot-pound region. Their primary goal is the number of shots they can get at around 25 foot-pounds.

Where does fill pressure fit in?
In all of this, the fill pressure hasn’t been mentioned, but it has been in the backs of all the minds working on the primary goals. For example, most airgun manufacturers know that shooters in the United States have easy access to air at pressures up to 3,000 psi. So, a gun that has a 3,000 psi fill limit is going to be received better than one that needs to be filled to the 4,350 psi that some German guns require. The United States is not a very big airgun market in general; but for guns that develop great power, it’s probably the biggest market there is. Anyone who builds an airgun of great power would be foolish to also build it to accept a 300 bar/4,350 psi fill. Doing so would cut out their biggest potential market.

On the other hand, the United States is a relatively small market for high-end 10-meter target guns — guns costing between $1,700 and $3,200. So making a gun in that category that also accepts a 300-bar fill isn’t foolish at all. If the European market for these guns is that much larger than the U.S. market and if it’s that much easier to get the higher pressure air in Europe, it makes perfect sense to build guns like that (for reasons I will discuss in a moment).

However, making a low-cost, youth-oriented target rifle that needs a 300-bar fill would be suicide! That’s because the world market for rifles in that category is located in the U.S. We have almost a million kids each year competing in 10-meter target matches, and that’s a number that’s been verified by the NRA. That’s why Daisy, Crosman and AirForce Airguns all have models in that category, and they know that to boost the fill pressure of their guns to 300 bar would be the kiss of death.

What about the boutique makers like Quackenbush and Barnes? They know their markets well and tend to build guns that fill to 3,000 psi or less. It’s true, many of the Quackenbush Outlaw Long Actions will accept higher fill pressures and still work well, but that’s an anomaly. They’re designed to use a fill pressure of 3,000 psi.

And what about the Korean precharged guns? Many of them will also accept a higher fill pressure and still function. In their case, like the Quackenbush guns, I think the design goal was 3,000 psi, but a too-heathy valve return spring boosted the top threshold beyond the goal. With guns like these, a shooter is rewarded for chronographing his shots and establishing what the actual fill pressure of his specific gun is before heading out to the field to hunt. He can get by with a 3,000 psi fill, but time spent on the chronograph will reward him with extra velocity for the shots he gets on each fill. What it usually won’t do is add more shots to the total per fill.

Can fill pressure be a primary goal?
While it normally isn’t a primary design goal, it is possible to make the fill pressure limit one of the primary goals for the airgun’s design. Tim McMurray and Larry Durham were the first ones to succeed in doing this in a very big way when they designed the USFT rifle. With a fill pressure limit of around 1,600 psi (these rifles all vary just a little on the exact pressure limit), they get 55 good shots of .177 Beeman Kodiaks traveling over 900 f.p.s. How is that possible? Well, there are 3 things that make it possible. One is a 25-inch barrel that gives a longer time for the pellet to accelerate, and another is a huge air reservoir that stores a lot of air at the recommended fill pressure. The third essential thing is a valve and hammer that are balanced perfectly to open the valve and hold it open long enough for the pressurized air to flow into the barrel and get behind the pellet.

Even before the USFT came about, Gary Barnes made one airgun that got a notable velocity on extremely low air pressure. With his outside lock airgun, Gary was able to get .25-caliber pellets over 25 foot-pounds of energy (a 20-grain pellet going 753 f.p.s.) on just 600 psi. That sounds incredible until you recognize that airguns were doing that and much more in the 1600s and 1700s. They had to because that was about as much air pressure as they could get into the gun!

Barnes’ outside lock gun did not shoot at a steady velocity. The first shot was the most powerful, and each shot thereafter dropped in velocity somewhat. But it was still possible to get 9 shots from his gun — all from an initial charge of 600 psi. His rifle has a 32.75-inch barrel, which is needed to extract every last bit of energy from the low air pressure. And the secret of his valve is that it’s held open not by impact but by a pair of cams that come into contact and are locked to travel together through an arc as the hammer falls. The length of the arc determines how long the valve remains open. So, the velocity of this gun can be regulated by the hammer spring, the valve return spring, and by the size and shape of both cam elements. It’s an ingenious design that dates back to the very early 1700s.

Knowing that low-pressure operation was possible for a PCP, I went to Crosman in 2006 with a proposal to design a low-cost, easy-to-fill PCP single-shot rifle. I’d been told by another airgun designer that my idea was impossible the year before, but I knew from the USFT and the Barnes gun that it was very possible. The result of that project was the Benjamin Discovery.

Why fill to higher pressure?
The next question we must ask is, if it is possible to get many good shots on lower fill pressure, and the Benjamin Discovery proves that it is, then why would anyone ever build a gun that takes a fill pressure that’s extremely high? Why, for instance, would a gun need to be filled to 4,350 psi? One good answer is the gun has a pressure regulator that drops the reservoir pressure to a lower pressure that the firing valve can handle. If you can drop 4,350 psi to 1,800 psi for a valve, you’ll get many more shots from the gun than if the valve had to operate throughout a range of pressures. That’s because no matter how a mechanical valve is designed, there will always be an upper limit at which it will work and a lower limit, below which it will stop working.

While such a system SOUNDS like the perfect design, you need to consider that the pressure regulator takes up space inside the reservoir that cannot be filled with air; and the valve also needs a small chamber of air at the lower pressure where it’s been designed to operate. That chamber also takes up some space where pressurized air cannot be stored. So, there’s a tradeoff when installing a regulator. You need enough room inside the reservoir to make the regulator system worth the loss of reservoir volume.

A more sophisticated system
A better way to use air at a higher pressure is to employ a computer-operated solenoid to open and close the firing valve. Then, you don’t need either the regulator or the air chamber. The electronic valve does all the work. If you also put a pressure sensor inside the reservoir that can tell the computer how much pressure it has to work with, the computer can calculate exactly how long the electronic valve has to remain open, giving you shot after shot at the same velocity, even when the air pressure drops by 2,000 psi. This is how the Benjamin Rogue works.

While the .357-caliber Rogue hasn’t been a huge success in the marketplace, the proprietary way its valve operates gives Crosman an edge in the PCP market. If they were to put their valve on a smallbore rifle built for general hunters and shooters, and if they could keep the price under control, they might just have the next generation of sporting PCPs. I don’t mean the gun has to be cheap, either. With Air Arms sporters selling for $1,000, and both FX and Daystates pushing $2,000, there’s plenty of room for a premium-priced Benjamin sporter with the Rogue-type electronic valve. It can’t look like it was sired by a boom box and a black rifle, and it has to be quiet, have a deadly accurate barrel and a trigger to die for; but such barrels are available, and Crosman knows how to make good triggers and quiet guns.

So, GunFun1, this is my answer. Fill pressure isn’t normally the first thing on the minds of airgun designers. Many of them just build guns that work, and the fill pressure falls in place toward the end of the project. Those who are more savvy know what the market will allow, and those who are really sharp know what the market is looking for.