I was missing the o-ring on the bolt of my 22mini and a couple of the kind members here jumped in to help. I believe I saw a list of the o-rings on one of the posts so I think your best bet is search.
@jwrabbit123, two fellow forum members own Evols. @dairyboy, and @mtnghost, maybe they can help? Or search their post. I believe they listed Oring sizes.