We use a shop press to compress the urethane races enough to get the snap ring into the groove. You can do it with a chunk of all thread and some creative socket use. Remember, you are trying to squash the race so you need the side away from the groove to be working against the barrel of the JJ. Otherwise, you can compress the races and the groove still won't be fully exposed.
So on the side away from the groove there should be a socket similar in diameter as the barrel itself. Then on the same side as where the grooves are you can use a socket slightly smaller than the diameter of barrel, but enough space to get the snap ring to fit over the socket and into the barrel. Probably good to use a washer to provide pushing to more area of the bushing. Obviously a nut and bolt as well. Sweet! Thank you for this one!!!