You don’t see it talked about much in the airgun forums, but many spring-piston air rifles and air pistols – springers – actually burn some of the lubricants in their compression cylinders during the shot cycle. Don’t worry; it’s a normal thing.
Here’s how G.V. Cardew and G.M. Cardew describe it in their book The Airgun from Trigger to Target: “The combustion phase is the phase in which most high powered sporting spring rifles operate. As the piston comes forward on firing, the temperature of the air in front of it rises with the pressure; this very high temperature causes oil, or any other combustible substance to burn, thereby increasing the pressure further, producing enough energy to drive the pellet up the barrel at a very high velocity.”
Further, they proved that the combustion takes place through an ingenious test that they called “The Nitrogen Experiment.” Starting with a .22 caliber Weihrauch HW35, they stripped it, degreased and rebuilt it with the correct amount of lubrication everywhere. They then fired it through a chronograph until it settled down at 636 fps with a 14.4 grain pellet (12.9 fp of energy at the muzzle).
They then placed the HW35 and a supply of pellets in a long plastic bag and sucked all the air out of it with a vacuum pump, leaving it sitting under vacuum for half an hour to remove all oxygen from within the seals and mechanism. The bag was sealed around the barrel and a rubber bung pressed into the muzzle to prevent oxygen from re-entering the gun. After that, nitrogen, an inert gas that does not support combustion, was blown into the bag to make it a manageable size for shooting the gun. The bung was removed and replaced for each shot, and a number of shots were fired. With the HW35 unable to enter the combustion phase of the shot cycle, the gun managed only 426 fps or 5.8 foot-pounds. The Cardews had proved conclusively that combustion is necessary for the proper operation of a sporting springer.
So, a little bit of lubrication is necessary so that combustion can take place. But what happens when your brand new airgun has a little too much lubrication? Check out the chart below.
This is the graph of velocities of an airgun that has too much lubrication and has entered into what the Cardews call the “detonation phase,” or what airgunners generally refer to as “dieseling.” Instead of making normal shot-cycle sounds, the shot goes off with a bang, producing the wild variations in velocity that you see above. Often smoke comes out the barrel and there is a characteristic smell. In severe cases, dieseling can actually bow out the walls of the compression chamber and drive the piston backwards with such force that it kinks the mainspring.
Fortunately, it is usually the case that a handful of shots with extra-heavy pellets will drive the excess lubricant out of the powerplant and settle the airgun back into normal operation. Below is the velocity graph of the same airgun after it was shot enough to settle down.
The bottom line: high powered sporting air rifles and air pistols require some combustion of their lubrication to operate properly. But there is such a thing as too much. If you find your air rifle or air pistol dieseling, 5-10 shots with the heaviest pellets you have of the appropriate caliber may help to correct the situation.
Til next time, aim true and shoot straight.
– Jock Elliott