Jon,

     From my admittedly limited experience with Maynards, I would be concerned not just with the size of the firing pin (which is a significant factor) but also with its retention.  Just how was that gunsmith conversion accomplished?  The factory conversion was not as strong as a modern rifle, and I am not too sure what variations on the theme may have brought to the table in the way or strength either.  I'm too fond of my right eye to want to have it visited by a fast moving fragment of a firing pin assembly! 

    I think the original design of the conversion would probably handle .25-20 target loads in smokeless, but I would do a LOT of careful testing before I trusted my face behind it.  Have you tried it out yet with BP loads?  If not, I think that is where I would begin, then progress very gingerly toward light smokeless loads of perhaps 2400.   I do know that there have been some .25-20 SS shells made with large rifle primers...I saw a few at Etna Green a couple of years ago...but availability would obviously be a problem.  You might wish to contact Dave Casey about some specially turned cases that would include the large rifle primer, or you might make a new firing pin whose nose section is reduced while keeping a shank that fills the hole through the standing breech.   

     Just some random thoughts that are probably worth about what you paid for them, but I am very interested in this project and would love to hear how you progress with it.

All the best,
Froggie

Sounds like a plan, Jon.  I am desperately hoping to make it to at least a couple of the Thurmont matches.  Even with my scope problems on the second day, the one I attended this past year was one of the high points of my '06 shooting season.  In the meanwhile, if you feel like taking a few pictures showing the screw and collar arrangement as well as the breech face where the firing pin protrudes, I would be happy to try and analyze what we are working with.  You might wish to send them to me directly using "gfrog53ATyahoodotcom" (address properly edited of course.)  From your description, I am thinking that maybe that arrangement would be as strong as or even stronger than the original factory design, particularly if the collar is indeed threaded into the threads where the percussion nipple went.

BTW, I am building a .25-20 SS low-wall using an original smooth side receiver and a Green Mtn barrel, and already have a high-wall with what seems to be a late factory rebarrel in that caliber, so I have a lot of loading and casting stuff for this caliber if you need any help on those lines.  Stay in touch!

Froggie