O-Ring Size for Hammerli Skinny 480 Air Pistol Cylinder?

[Gwhite]

I'm helping coach the MIT pistol team, and they have a couple of Hammerli 480's with the newer long straight sided air cylinders.

One of the students filled their tank today, and it leaked most of the air when she replaced it on the pistol. The seal on the little projection that fits into the pistol appears to have died. We had a new cylinder that I gave her to use that had a small but otherwise normal looking O-ring on it.

Does anyone know what size the O-ring is? As a guess, it's about 7mm OD with a 1.5 mm cross section. I need to order some O-rings for other pistols, and would like to add a bag of these to our collection.

Thanks!