by B.B. Pelletier

So much of today’s airgunning depends on O-rings, yet how much do any of us know about them? Today, I’d like to tell you more and cover a few basics.

Different materials for different jobs
When CO2 cartridges were first used in airguns in the late 1940s, the choice of materials they could be made from was very limited. Most early O-rings in guns were made of a material that was porous to CO2 under pressure. After being in a gun and charged for hours or even days, the O-rings were swollen with CO2 gas! They were several times their normal size, which made it all but impossible to open the gas chamber to install another cartridge after the old one ran out. You had to set the gun aside for hours, to allow the gas to slowly leave the O-ring. If you didn’t wait, you would tear the O-ring by unscrewing the cartridge chamber when the ring was still swollen.


The Schimel was notorious for having O-rings that swelled!

Besides the American-made Schimel, guns made in others countries had this problem until very recently, with, perhaps, China being the worst offender of all. Now that you know about this problem, never try to unscrew a powerlet chamber when the cap feels extra tight. You might just encounter a 1950s problem all over again.

Hardness is another factor
The Shore durometer test gives you the hardness of rubber and other substances. The test measures the O-ring’s resistence to indentation from a known force. The O-rings we use are measured by the Shore D scale, which is for harder materials. You hear the phrase, “90 durometer” kicked around a lot by airgunners these days. A 90 durometer reading is VERY hard, but not all airgun O-rings are that hard. In fact, a lot of them wouldn’t work right if they were that hard. There is also a less well-known Rockwell hardness test for synthetic materials, but the Shore durometer rating is the one we use most.

Here are some O-ring facts.

1. To perform correctly, a hard O-ring needs closer tolerances than a softer O-ring.

2. An O-ring usually needs lubrication to do its job – but not always.

3. When an O-ring seals something, it only needs to be finger-tight.

4. An O-ring can look fine yet hide a tear or a puncture and leak under pressure.

5. An O-ring can look ratty yet still seal perfectly.

6. The durometer rating of an O-ring can change over time.

The seats or channels they sit in help O-rings work!
If the seats are too wide or too deep, the O-ring will not seal the joint as intended. Also, the shape of the O-ring seat or channel is somewhat important. While there is a lot of room for slop with an O-ring (that is one of their endearing qualities), you can’t get away with murder. A perfectly square channel with no radius in the corners may present sharp edges to the O-ring under pressure. In other words, it can cut the O-ring, causing it to fail quickly!

Sizes are important, too
Not only do O-rings come in different diameters, they also come in different thicknesses, and that dimension is just as important as the diameter. O-rings are available in both metric and imperial measurements. Sometimes, you can cheat and use a metric ring for an imperial application, just like a 14mm wrench will work when a 9/16″ isn’t available.

Properly designed O-rings seldom wear out
As long as they’re lubricated correctly (if they require it), O-rings can last a very long time. In some applications where they abraid as they are used, such as the seals around caps that are constantly opened and closed, they may wear out sooner, but I’ve seen O-rings that have lasted 25 years and are still going strong. If the material was chosen wisely and the seat is correct, an O-ring can last and last.

The magic word is silicone
It’s hard to beat silicone as an O-ring lubricant, unless the equipment specifically warns against using it. An arigunner should sock away some pure (food-grade, which is so pure it can be used in food machinery) silicone grease and a bottle of silicone oil. I have said this before, but nothing reseals leaking CO2 guns faster than a drop or two of Crosman Pellgunoil.