This report covers:

  • Good or bad?
  • Velocity
  • Longer barrel
  • Barrel length on a spring-piston airgun
  • Other mods
  • Stage one tune
  • Airgun equivalent
  • Last mod — new stock
  • Summary

Today I address a subject that I suspect is on the tip of many tongues in the airgun world — modifications. 

Good or bad?

I’ll tell you up front that some modifications work well and others do not. Those that are sold as kits by reputable dealers like Pyramyd Air or manufacturers like Vortek, are the safest bet. You have watched me install several modification kits in this blog and if they were these kind of kits the results were always spot-on.

There is no all good or all bad when it comes to mods. So, it’s worth considering what works and what doesn’t.


Many airgunners want more velocity. Some want to shoot heavier projectiles at the same velocities at which their guns now shoot light projectiles and that’s the same thing. What works?

Longer barrel

A longer barrel will increase the velocity of a pneumatic or CO2 gun — to a point. That point is somewhere around 24-28 inches of barrel for a pneumatic and 14-18 inches for the gas gun. For a big bore PCP the limit might be a few inches longer. Beyond these points you reach the area of diminishing returns, where you may get a few more f.p.s. at the cost of a barrel that’s way too long. And, in the one test I once did with a CO2 rifle, the velocity actually diminished after about 18 inches of barrel, as I recall.

But, wait a minute. There is more to it than just the length of the barrel. It’s also the valve that the air or gas flows through and the chamber that is behind that valve, if there is one. You can’t add a long barrel to a CO2 pistol and expect much velocity gain unless the valve is also modified in some way. And a longer barrel on a multi-pump pneumatic like the Benjamin 392 can only use the air that’s in the gun. For example, adding three inches of barrel to a 392 won’t increase the velocity on three pumps of air as much as it will on 8 pumps. So barrel length goes hand-in-hand with the valve, plus the space behind the valve where the high-pressure air or gas is coming from.

Barrel length on a spring-piston airgun

A longer barrel can speed up a pneumatic or gas gun. But a spring-piston gun is different. For them the barrel needs to be made shorter — to a point. For instance, the barrel on an Air Arms TX200 Mark III is about 9 inches. It looks longer because there is a steel jacket around the barrel that holds the baffles that silence the report. But the actual rifled barrel is just 9 inches long, more or less.

In the 27 years I have been writing about airguns I’ve heard of several people who have cut off the barrels of breakbarrel springers in the hopes of increasing velocity. That never turns out well. I often find out about it when they ask me if I know of a source for a barrel for a certain air rifle. Dragging the story out of them takes time but in the end they were after more velocity because they heard that a breakbarrel produces its maximum velocity in the first 6 to 9 inches. After that the pellet is coasting, and in their minds, slowing down.

According to tests done in the 1970s by the father-son team of Cardew there is truth to the short barrel being all that’s needed for a spring-piston powerplant. The piston in those guns compresses a small amount of air that hits the pellet with a blast. After that, the air pressure keeps dropping, the farther down the barrel the pellet goes. With a pneumatic if the valve stays open after the pellet starts on its way the pressure behind it may drop, but it still remains higher than ambient air pressure. It’s like pushing a car. It takes a lot of effort to get the car started but once rolling, not as much push is required.

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Other mods

I’ve addressed velocity. What are some other modifications that can be done to airguns? Before I get into that subject, though, how about one from the world of motorcycles — specifically Harley Davidson.

Stage one tune

A stage one tune for a Harley engine consists of a less restrictive air intake and exhaust and a tuneup for the electronic brain that runs the engine. It means more fuel is available and the engine runs less lean. Since no internal parts are changed, the stage one tune is very popular.

Stage one tunes tend to lower the operating temperature of most Harley engines, and Harley engines do suffer from high operating temperatures. And this tune is recognized by and even promoted by the Harley Davidson Motor Company.

But I met a guy who ripped out the fuel injection on his Harley and replaced it with a carburetor. He told me he felt the engine ran better with a carb, though he did admit that he had to adjust the carb often. Same guy told me that after 11,000 miles he needed a new engine. He said that to me with a straight face!

That would be my example of a mod that doesn’t make any sense — just like when BB Pelletier had the catalytic convertor cut out of his 1976 Ford Pinto and a straight pipe welded in so he could run leaded gas. Two years later old BB couldn’t register his Pinto in a new state, and he lacked the money to have the catalytic converter reinstalled. Duh!

Airgun equivalent

What’s the airgun equivalent to a stage one tuneup on a Harley? How about a Vortek PG4 SHO kit for your HW 50S? It’s gonna shoot better, have less vibration and cock with the same effort. BB has one of those kits on hand and plans to make that switch for you in this blog.

The opposite to that would be the guy who has a “special” tune for the valve on your Benjamin Marauder. He claims you’ll get more shots and higher power. When you make the switch and don’t get either one he tells you, “Well, I did!” When you want to switch back to the old valve you discover that the guy has machined a part of your gun that renders it incapable of switching back.

Last mod — new stock

BB did this one in front of all of you. His new-style Benjamin 397 stock was shaped so he couldn’t see the open sights on the rifle. You got to see that in Part One of that report.

Benjamin 397
The new style Benjamin 397 has a synthetic stock with a very high cheekpiece.

397 maple finished
Reader Hank made this gorgeous curly maple stock for BB’s 397. Now BB can use the peep sight on the rifle and he is proud of the way this rifle looks.

A new stock seems like a less risky modification than many others I mentioned — unless you are modifying the old stock. Remember — you can’t uncut wood!


No matter what, at some point many of us will want to modify some airguns. I just started the discussion today. It’s now time for you to talk among yourselves.