i've been using my FR1 on my cassette lockrings for years. it needs a little encouragement at first but, if you don't tighten the heck out of your lockrings, in the first place, it shouldn't take much to get the FR1 in a millimeter or so to get the lockring off. mine works now on both cassette lockrings and shimano freewheels.

it's not for the faint of heart. there's some finesse involved... some ungenerous types would call it butchery.