I have a Mk II Competition model that I used to shoot bullseye with for years, and one of the best things I did was put a full Volquartsen kit in it. I have not tried this on a Mark III, but on my Mark II I had issues with combustion products building up in the lockwork and causing the trigger pull to get crappy until I cleaned it. I found the info on making a shield to prevent it, and it made a big difference.

http://www.1bad69.com/ruger/blastshield.htm