What Jim Watson said; good info there. Also, make sure you don't have a leading issue. Just a very few shots leaving lead in the bore will totally destroy accuracy. Shiloh Creek Bore Solvent works very well at removing lead. Tight cotton patches, using the proper jag, well-wetted with the solvent, will let you know quickly if leading is a concern in your rifle. Use a bore guide, too. I've literally used a rubber hammer to drive the rod through the bore to get the lead out.
I have a 45/90 that I shot "as cast" bullets in it when I got it, and it leaded so badly, that after 3 shots, my accuracy at 50 yds. was like a shotgun pattern (7-8 inches off the bench). Those bullets were right at .459" out of the mould. When I sized them down to .458", the gun started performing, and very well I might add, and NO LEADING; problem solved. That bullet is a Buffalo Arms, Creedmoor, 540 grainer.
I also have a 45/70 Shiloh Sharps, and I use only black powder in it for the charge (my 45/90 gets only black, too). I don't have to size the bullets in that one, and it shoots wonderfully. What I use for a load in that one is:
Fire-formed WW cases (case length is 2.95"), CCI BR-II primers, Lyman Postell bullet "as cast" using a 20:1 lead/tin alloy, 68 grns. of GOEX 2fg black powder. Though I can't remember the compression measurement, the bullet is seated out of the cartridge case to where it engages the rifling about half-way through that first driving band on the bullet when the breech is closed. I use a .030" veggie wad over the powder, and a newsprint disc over that, which the base of the bullet rests against. I also use a newsprint disc over the primer on the inside of the case. Be sure to use a good lube on the bullet, too. On this load, one grease groove is exposed on the loaded cartridge. Oh, yeah, cases after fire-forming are neck-sized only. I use a Redding neck-sizer, and a Buffalo Arms custom expander plug. I size only as much as the bullet seats in the case.
Your O.A.L. using this load in your rifle will determine how much compression you'd use (get a compression plug; don't use the bullet to compress the load), and, with the bullet engaged in the rifling as before mentioned, your O.A.L. will be determined for your rifle. Good blow-tubing is also a must with black powder loads.
Hope this helps a bit, and good luck with working up a load for your Sharps!
P.S. I also use a Lyman mould (457193) that casts a bullet @ 410 grns. using the 20:1 alloy. It also shoots very well in my 45/70, and I've used it for hunting with excellent results.