No, its not a side mount, the only difference is the rimfire barrel has a flat top as opposed to being rounded like the other Encore barrels. If you look at the pic a couple posts up, I believe by doc, you can see that the mount is incorrect for the rimfire barrel, and is probably the mount for a regular Encore barrel. I'm thinking the barrel being offset is of minor consequence, at least in my experience, I don't think I've had such accuracy w/rifles. I couldn't be happier with it. I haven't had the opportunity to try "good ammo" in it yet, and really don't have a reason to at this point.