Hi. Those mainsprings can be a real bear to compress. I've worked on a couple guns (Fox Model B, Nitro Special) and the way I've compressed the springs is by using a piece of wood with the profile of the rear of the hammer cut out of it. The piece of wood I use is maybe 3/16" thick, 2 inches wide and I cut it to about 5-6" inches in length. Anyway, I used the hammer as a template and cut that pattern out of the end of the wood. I then put the hammer in position on the spring/cup and use the wood to push down on the hammer far enough to get an alignment punch in the hole. The receiver is clamped in a padded vise during this. It helps to have a helper and cover the end of the wood you're pushing on with a rag or something cause you end up pushing down real hard.
I'm not at home so can't post a pic of the tool/wood I use. Not sure if I described it well enough. The alignment punches work well to grab the hole in the hammer. If you monkey around with it long enough you'll get it.
I'd be happy to clarify this if you have any other questions.
Vince
