Thanks, everyone. The smallest size I've tried is:
and it's damn small and hard to get on, but still too big. Mine are hard, though, 70 durometer, maybe something softer would mash flat and make it into the bore.
The 007 is a size bigger, guess I'll try a 005.
Hey Bill... if there is one fundamental truth in rocket science, it's getting the right size oring, believe me.