"sirk"Dear Omaralla, why dont you just measure it with Vernier Calipers, its very easy:
first number is the internal diameter, second number is the thickness. sounds like your supplier selling metric which is good, you are not that way off.
AS, BS numbers work different way so stick with the 10x something in metric way of looking at it.
If you change the thickness you may have to change the ID too, so do not measure the wrong size new o ring but measure the old.
I feel for you guys, it is very bad for owners that FX wont give you the numbers, it should be in the manual, what is in the manual, warranty and safety poop only?
Any spare o rings come with the gun? if it is a caliber changer, definitely would belong to the package.