Tuning AEA HP .22 cal breech o-rings?

I'm not sure on the size, but if you have the original you could take it to a hardware store and size it there, or you could ask some other dealers what the size is and order some from the oring store online. If they are willing to tell you the size I would ask them the sizes for all of the o-rings in the gun that way you can order spares of all the sizes you need. I have bought a few Buna oring kits that where recommended so then I have a stockpile of all the different sizes that I would ever need. Sorry this isn't an exact answer but will hopefully help some.