they all say they always thought they all had octagon barrels.
They made them Octagon, Round, and half Round and half Octagon. The only model I know of that only came with a full round barrel was the 1874 Sharps Business Rifle, with standard double set triggers.
My WAG on the caliber would be the 40-70 Sharps necked, a bottle neck cartridge. Sharps usually marked their barrels for bottle neck cartridges with just the caliber, 40-70 straight wall case chambered rifles were marked .40 -2.5 , the 2.5 meaning a 2 1/2" long case.
A chamber cast is a must to determine exactly what it is. Sharps would chamber in Winchester calibers too, and many were sent back and re-chambered for a larger case caliber. When you know the caliber, I can track down some ammo for you.
Get some pics up and I can give you a guesstimate on value, even wallhangers will bring $2K. How is the bore condition?