by Tom Gaylord
Writing as B.B. Pelletier

This report covers:

  • More power!
  • Example
  • Stronger mainspring?
  • Piston stroke
  • Increase the piston length
  • Dual power is possible through piston stroke
  • Larger piston?
  • Cost
  • Transfer port size
  • Port location
  • Piston weight
  • What can be done with this knowledge?

Today’s topic was suggested last week by reader Riki from India. A lot of other readers jumped on the bandwagon when he asked for it, so I agreed to write a series of reports. The question is — how do airgun manufacturers control the power/velocity output of the guns they make?

More power!

An American airgunner who is new to the hobby will look at this in a different way. He will wonder how airgun manufacturers get the highest possible velocity/most power from an airgun. He won’t appreciate that in nearly every country in the world other than the United States the governments have limited the power of airguns. And there is no common way they limit it. In the United Kingdom they limit the output by energy, allowing no more than 12 foot-pounds for air rifles and 6 foot-pounds for air pistols, I believe. They aren’t concerned with velocity, except as it produces energy. This is a thoughtful regulation that forces airgunners in those countries to learn basic ballistics. It also forces manufacturers to test their airguns with almost every pellet to be sure they are not exceeding those limits.

Spain and Germany also limit airguns by energy, though they vary widely. Spain allows around 17 foot-pounds, I believe, while Germany is just over 5 foot-pounds.

Other countries like Denmark and Canada limit airgun velocity rather than energy. That’s a more simplistic way of doing it and it does allow the larger calibers to slip past with energies sufficient to kill deer and similar game. But the point is, no matter what country you are in, with very few exceptions (New Zealand?), airgun power is limited.

Where this comes into play is when a manufacturer wishes to sell their airguns in different markets. They must learn what those markets permit and not only produce guns that meet the specifications, they also have to ensure that their guns cannot be easily converted into an illegal configuration for that country.


Let’s say Crosman makes a breakbarrel rifle that’s very popular in the United States. It’s accurate, easy to cock and has a great trigger, plus the price is just under $300. It gets 1,100 f.p.s. in .177 caliber with a 7-grain lead pellet. The U.S. market will be good for around 10.000 sales over the next three years. That’s good, but if you could sell this rifle in the UK, the sales potential would almost double. Canada, Germany and the rest of Europe are pretty much out of reach for this gun without a major redesign. But is it worth it to try to reduce the rifle’s power from 1,100 f.p.s./18.8 foot-pounds with a 7-grain pellet to 12 foot pounds/879 f.p.s., for the UK market? A bunch of airgunners sitting around and having a bull session will probably say yes, but do they know what that involves? That is what today’s report is all about.

How do airgun manufacturers control the power output of the airguns they make? Today we will look at spring-piston powerplants.

Stronger mainspring?

The novice response to gain more power is to install a stronger mainspring, and of course the reverse would also be true. But it often doesn’t work as you might think. Not only does a more powerful spring usually not increase the power/velocity, it almost always decreases it, while increasing the effort to cock. A strong mainspring has a place in spring gun power, but it is one of the last things to consider. Several things are far more important.

Piston stroke

One of the easiest ways of controlling spring piston air rifle power, all other things being equal, is by changing the length of the piston stroke. Hot-rodders knew this way back in the 1940s, but airgun designers didn’t catch on until the late 1990s. Now they all know about it. The longer the piston stroke the greater volume of air that gets compressed and the more power it produces. But there are several ways to do this and not all of them work well.

Increase the piston length

I have read where people advise others to just put a piston extender between the piston and the seal to reduce the length of the stroke and decrease power. This can work (though some alteration of the cocking slot may be required) but it can also be easily overcome by removing the extension, and customs inspectors are wise to it. They may not catch a single gun coming into the country, but they will certainly stop an entire shipment of airguns that have this feature. You see, customs inspectors are used to being fooled, tricked and hoodwinked. There isn’t much they haven’t seen. No reputable company would ever try to pull the wool over their eyes, because Customs has the power to seize entire shipments and fine shippers for violations.

The better (read more acceptable) way of limiting piston stroke is to redesign the piston and cocking linkage so the stroke is permanently reduced. Diana inadvertently did this in the 1970s with their Diana model 35. It’s a large breakbarrel rifle that looks like it ought to be a powerhouse, but the short piston stroke limits its capability, with no easy fix. When the model 34 came out a decade later, it was vastly more powerful — just from having a longer piston stroke.

Dual power is possible through piston stroke

Since the piston stroke does control the power, it is possible, through design, to control and vary the power on a gun that way. The Beeman P1 pistol is such a gun. In .177 and .20 calibers the P1 has two power levels. They are selected when you cock the gun. Stop at the first place where the piston is caught by the sear and you have low power. Continue to cock the pistol to the second and final detent and the power will be greater. This is a dramatic demonstration of how piston stroke affects power.

Larger piston?

Okay, BB, I get it. Longer stroke means higher power. But, what about a bigger bore?That’s also what hot-rodders do — they increase the bore size of their pistons.

Yes, but hot-rodders don’t have to fit the engine block to their shoulders. A bigger bore does increase power just like a longer stroke, but when you get over a certain size, the spring tube becomes unmanageable. Stroke length is the best way to increase power and piston diameter is second best.


Before we move on I just want to say that, while stroke length is a great way to control the power of a spring gun, there is a cost involved when you want to make a change. The individual airgunner thinks it’s just the cost of the parts, and as far as he is concerned, that’s all it is. But the manufacturer that wants to produce those parts has to look at the additional engineering that’s required to make the changes.

The piston and cocking linkage will have to be changed at a minimum, but usually changing those parts necessitates a new mainspring, as well. Instead of $90 for just the new parts for each airgun (yes, the other parts do come out, so the cost of the gun remains the same), think about $35,000 to design the changes, prototype the parts, test the prototypes, make additional changes, test those, baseline the software for the new parts, including cataloging the parts and their new numbers to keep the inventory straight, making catalog, advertising and online changes that differentiate the new guns of different power but the same model name (remember the 4 power levels of the Diana Mauser K98?) and any legal fees involved in getting the new guns accepted by the state departments/home offices (and their customs departments) of the various nations to which you wish to export. Did I say $35,000? Perhaps even more!

Are there other ways of controlling spring gun power? You bet!

Transfer port size

There are two ways to LOWER the power of a spring-piston airgun using the air transfer port. First, make the air transfer port smaller and second, make it larger. It turns out that the air transfer port is usually optimized for a particular model of airgun, although caliber changes in a specific model does blur this optimization a small amount. What I mean is — a certain springer in .177 may need a port of a certain diameter, while in .22 caliber in the same gun a slightly larger diameter port might work slightly better. The differences I’m talking about are extremely small, thousandths of an inch, which is why when companies make a certain model. the transfer port is the same size regardless of caliber. Nobody can afford to make three different spring tubes for one model airgun just to optimize the transfer port size.

When I wrote The Airgun Letter and also the Beeman R1 book, I did some testing that demonstrated that a port size of around 0.125-inches or 3.175 mm seems to be a good size for many guns. If you want to read more about this I extracted the chapter of the R1 book that deals with transfer ports and put it into a report.

I once saw a production port as large as 0.150-inches which is 3.81 mm, but that was on a mega-magnum gun that had a huge piston. I’ve also seen ports that were smaller than 0.125-inches, but not by much. However, port size isn’t the only thing.

Port location

Where the transfer port is located in the airgun also makes a difference, when it comes to power. A port in the center of the compression chamber that flows straight to the breech is the most efficient and will give the highest power. A port that is located in the center of the compression chamber and then angled to the breech, or a port located on the edge of the compression chamber end and angled to the breech is less effective. However, these port location choices are never made to control power. They are fundamental design choices for the entire airgun. But when a company like Air Arms puts a central port in the chamber of a rifle like the TX200 Mark III, they demonstrate how effective it is.

Piston weight

When the weight of the piston is changed you don’t really change the power as much as you bias the gun to lighter or heavier pellets. Heavy piston equals heavy pellets and so on. All it does is keep the piston from bouncing off the cushion of air that it compresses.

What can be done with this knowledge?

Here is where Mr. Wizard wows the kids. Knowing all the above, you can build a spring piston airgun that’s easy to cock because it has a light spring, yet it generates lots of power because of a long stroke. The TX200 approaches this ideal, but more can be done, I believe. Pay attention to dampening vibration, and such a gun would become a world-beater.

Or, a dual-power spring rifle could be built. It could generate low power (5 foot-pounds) by stopping at the first detent and high power (18 foot-pounds) by stopping at the second detent. Like the P1, power would be controlled by the length of the piston stroke. By using a progressive rate mainspring in conjunction with the cocking mechanism, the cocking could be very light (15 lbs.) for low power and heavier (30 lbs.) for high.

I hope this has answered some of your questions about how spring-gun power is controlled. We will look at pneumatic powerplants next. I might be able to do a third report on CO2 powerplants, though I haven’t decided yet.