Great mod, really innovative to expand the plunge tube and shaft like that. Although when it's fired it sounds like there is a lot of room for the spring to move around in, maybe you can add another spring, or a piece of one to use up this space.
Everyone would has their own opinion on how hard a mod is, so all results would be different. Perhaps a sort of poll, on each write-up for people who have completed the mod. Where they could rate the difficulty of the mod on a scale of 1-10.