They are heavy, the actual surfaces are generaly well finished, you are not likely to improve on them.  Don't change the notch or sear relationship either . Make sure the inside of the lock is clean. 

The Trigger spring is the key to lightining and not too hard to find at gun shows. Most Springfield locks use the same one even the Muzzleloaders. If you can get another to play with you can lighten the spring by reducing it's width.  It does not take much to make a pretty big difference so go easy when you grind. It's easy to overdo it so make sure you have another before grinding.

Do you know how to dissamble the lock ? back off a little bit on the lock screws and tap them with a wood hammer. If you only back off a little there is less chance of spliting the stock when  the lock plate  comes out from the inletting. 

Also the Arsenal combo tool is made to remove the main spring. Cock the lock put the jaws of the combo tool on the main spring and let the hammer down. The tool will hold the spring compressed and the hammer sear and all will come out easy. If you don't have one you can do it with a small adjustable wrench.

Boats

Sounds like your friend may have reduced his sear engagement without the corresponding, and absolutely necessary, reduction in sear spring tension. If you don't reduce the tension along with the engagement, then frequently the trigger will kick back after disengaging, and will land on/in the half-cock notch and break it.

Limited sear engagement is fine and quite safe as long as the springs (mainspring too) are balanced along with the engagement and everything else in the lock mechanism. Your friend needs to take his lock to a good smith & pay a few bucks, a 3-lb safe pull is very achievable by even an average workman who knows single shot rifles. That's not the only solution of course, but since your friend has already broken one part.............
Good luck, Joe

Lee shaver has worked on several trapdoor locks for me an I am very satisfied with the results. There are also gunsmiths who do a lot of trigger work for competitors in the NSSA who could help you out. I built a sporter once with a set trigger and had nothing but trouble with it. The lock is simply not conducive to a set trigger, it can be done, but will be expensive. I would send it to a good gunsmith who knows and is experienced with musket and trapdoor locks. Joe S

I've seen vintage photographs of modified Springfields. Some of them had a poured light metal nosecap, a new trigger guard of a squared sort of configuration, and double set triggers. If you are not happy with existing triggers I believe installing an inexpensive set designed for muzzleloaders would not be too difficult.

Dick

Joe,
   Thanks for the reply and your experiences. Adding an extended tang to my rifle is out of the question as it's a finished piece. I too found that a canned set tirgger set up wouild be impossible to mount to the oringinal trigger plate because of the rear screw.  I was hoping someonw made a single set trigger, but I have not found one yet. I believe the way to go is to modify the sear and trigger spring and get the best pull you can with the original parts.