This report covers:

  • The nitrogen experiment
  • Dieseling
  • What the Cardews’ experiment really means
  • Please forgive
  • Detonation
  • Intentional detonation
  • What does BB do?
  • Heavy pellets
  • Summary

Today we’re going to take a look at dieseling and detonation — two things that many shooters believe are bad. One of them is, and the other is inevitable.

Let’s begin with the comment of a new reader, tomek from Germany, who is Polish-born. He mentioned his Suhl 300 air rifle. I’m not familiar with that airgun, but it sounds like a neat little breakbarrel that Haenel might have made. 

Regarding dieseling, here is what tomek said.


My experience with diesel effect: if you have a detotanation-like events you should dismantle the system and clean it right away. Waiting until the grease or oil will burn out is not a good idea. I own also a HW50 and once, due to my mistake, I had a really bad detonation which broke the spring and destroyed the piston sealing.

There is no point starting with chrono measurement if you can even smell burning grease. If it detonates – not at all… I’m pretty sure if you would clean the system and make a proper -grease finish again with only a silicone-oil film in the compression chamber the chrono results will be different.”

I used to feel the same way, until I read the experiment done by the Cardews — father and son, who wrote the book The Air Gun from Trigger to Muzzle.

The nitrogen experiment

The Cardews tested a .22 caliber Weihrauch HW35 that was getting 636 f.p.s. with a 14.4-grain pellet. They put the rifle in a large plastic bag and sucked out all the air. The left it that way for 30 minutes to get all the oxygen out of the piston seal, then they filled the bag with nitrogen, which doesn’t support combustion. The muzzle was poking out of the bag, but they resealed it with a plug after each shot. They shot this gun in a pure nitrogen atmosphere for several shots and recorded a velocity average of 426 f.p.s. The gun had only 45 percent of its initial power (energy, not velocity) when it was not permitted to burn fuel.

Then, they took the gun out of the bag and continued firing it and the velocity rose back to the initial figure. This is their proof that combustion happens with every shot. Not only that, but with some guns that have tight velocity spreads, it is also very well regulated!


What the Cardews were testing was dieseling, however, they also included detonations under the same title by labeling them “violent diesels.” The Cardews also did their experimentation at a time when 600 f.p.s. was considered normal for an airgun. This was in the early 1970s, when 800 f.p.s. was considered the maximum velocity a pellet rifle could achieve. It was even called the 800 f.p.s. “barrier.”

What the Cardews’ experiment really means

The Cardews don’t address this, but what their nitrogen experiment means is that in an oxygen atmosphere, something is happening that boosts the velocity of the pellet — and in many air rifles the boost is very consistent. In their experiment the boost was 210 f.p.s. What caused it?

They did determine that when the piston in the rifle they tested went forward, it stopped about 0.10-inches (one-tenth of an inch) or 2.54 mm from the end of the compression chamber. This stop was caused by the pressure of the air the piston had compressed to around 1,350 psi. That pressure caused the piston to bounce back off the high air pressure, which then lowered the pressure to around 1000 psi. That rebound was just 0.02-inches. That’s right, two one-hundredths of an inch.

When the compressed air expands from the piston bouncing back, the small oil droplets flash to vapor. If the adiabatic temperature (heat caused by compression and not long-lasting) remains high enough, this vapor instantly ignites, causing a diesel that’s no different from the one in Rudolph Diesel’s engine. This small explosion of igniting oil is called dieseling and it’s normal in spring-piston airguns. If it didn’t happen your 636 f.p.s  rifle would shoot at 426 f.p.s.

Please forgive

I am broad-brushing the work that the Cardews did so I can explain a simple point. Most spring-piston airguns diesel on every shot. However if they shoot very slow — perhaps in the 400 f.p.s. range and below, like the Slavia 618s we have looked at — then they may not diesel at all, because the temperature of the compressed air is too low to ignite the oil. But even in weaker rifles like the Diana 25 and 27 it undoubtedly occurs.

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The Cardews showed numerous pressure curves that showed the differences between a normal diesel and what they called a violent diesel and I am calling a detonation. Their term is more precise, because even the diesel is the result of a small explosion, or rapid oxidation of the air/oil vapor.

But the detonation is something more. A detonation is an explosion you can hear and even see if you shoot in a dark place. You will see fire coming from the muzzle of your rifle.

Now many believe that if you see smoke coming from the muzzle the rifle is detonating, but it’s not. My .22-caliber Diana 807s both smoke when fired and they have never detonated. A detonation may also have smoke, but it is the sound that is the determinant — either that or the chronograph.

Intentional detonation

Let’s look at how a gun can be made to diesel intentionally, starting with the famous Weihrauch HW Barakuda EL54. That was a standard breakbarrel rifle, the HW 35, to be specific, with a tube on the right side to inject a small shot of ether vapor into the compression chamber just before the shot was fired. The point is that manufacturers recognized the power potential of a detonation and tried to harness it to make airguns shoot faster. Today, we would call the EL54 a firearm, because that is exactly what it is.

You don’t have to be an airgun manufacturer to make a spring gun diesel. Now that you know how it works, you’ve probably figured out that it takes just some fuel coming in contact with the superheated, compressed air generated by the piston. Children have known this trick for at least half a century, which is the basis of the Oil-Can Louie story that’s in The Airgun Digest Volume 2. However, that story ends with the destruction of the airgun, so please don’t experiment that way.

Gamo sells .177 breakbarrel spring rifles that they claim are capable of generating 1,600 f.p.s. with PBA pellets. When a friend of mine chronographed the one he owned, it was shooting just over 1400 f.p.s. I asked for any reader with a Gamo Hunter Extreme to chronograph their rifle and to tell us the numbers. To date, no one has come forward. Gamo used to show a film clip on their website from the Shooting USA TV program that shows a shot chronographed at more than 1600 f.p.s. I tested a PBA pellet in a .177 Condor and it went only 1486 f.p.s., which tells me something might be fishy about Gamo’s claim. However, it is possible to make a pellet go as fast as that televised shot with some oil-can Louie trickery.

What does BB do?

So, why did BB keep shooting the HW 50S if it was detonating? Well BB has worked on many dozens of spring-piston rifles and has a sense of how bad something might be. That doesn’t make him omniscient — BB makes mistakes all the time. That’s where a good number of his blogs come from. But he has a sense of whether the detonation is terminal or not, and he presses through if he believes it isn’t. It’s a risk I’m willing to take because I have seen it so many times. You do whatever feels right for you.

I have seen rifles that wouldn’t stop detonating and those I had to disassemble to correct. Many Chinese mega-magnums were like that. If the HW 50S hadn’t stopped detonating when it did I wouldn’t have continued shooting it much longer.

Heavy pellets

I have found that heavier pellets generate more back pressure and that stops the dieseling in guns that are going to stop all on their own. That’s why I started shooting RWS Superdomes after the lighter Falcons detonated. It stopped the detonations in about 4 shots.


That’s what I can tell you about dieseling and detonation. Now you talk among yourselves and we’ll all get smarter.