Your original 1863 Springfield has what is known as "Progressive Depth Rifling" which means that it can shoot undersize Minie Balls. For example, British Ordnance issued .568 Pritchett Balls (a Minie Ball without grease grooves) that was paper patched. Because there were so many complaints from soldiers about the ammo being hard to load, especially after they had fired a few shots in battle, British Ordnance reduced the diameter of their Pritchett Balls to .550 and found that they shot very accurately. You can do this ONLY with Pr0gressive Depth Rifling. PDR means that the grooves in the bore are deeper at the breech than the muzzle.
To further tighten your groups I recommend:
(1) Experimenting with various powder charges-generally there is one particular powder charge that muskets will shoot their tightest groups at. Generally it will be found between 45 grains of FFFg and 55 grains of FFFg.
(2) Weighing your bullets-I weigh and separate my bullets into plus or minus 1 grain weights.
(3) Recommend you size your bullets. Bullets pop out of the bullet mould at different sizes depending on the temperature of the lead and the temperature of the mould. Running your cast bullets through a sizer makes sure that all of your bullets are ALL the same size.
(4) Tight tang screw! Believe it or not, a tight tang screw does make a difference. Just tighten it up good and snug. That keeps the barrel stable in the stock during the firing cycle.
Go to
www.n-ssa.org/ and find a N-SSA team near you. They are a great bunch of fellas and will love to shoot with you.
Glad to help!!
GOOD LUCK
All My Best
Southron