If my memory isn't too faulty, I believe that the accuracy standard for Minie rifles like your Springfield was to place 60-70% of shots on a 6'x6' target at 500 yards...the area that a field piece occupied. These rifle-muskets were supremely accurate...when compared to the smoothbored muskets that they replaced. You should be using pure lead in those bullets, and a good lube. SPG has been mentioned, it's an excellent choice but a bit expensive for the big grease grooves on your bullets. When I was competing in Civil War Skirmish Association matches, I used a 50-50 mix of beeswax and Crisco. It worked well in my rifles, but was a bit stiff in colder weather.
David
