There were no Frank Wesson production cap and ball rifles, so I assume yours was either by Edwin Wesson or by Frank Wesson when he was working with Edwin or when he was in California. After his return, he established a business in his own name and began production of the so-called "double trigger" rifle (1859). But that and all later rifles were cartridge rifles.
Some good pictures would help, and the rifle sounds like a high quality target rifle. But with those key parts missing, I doubt the rifle has much value. It is not clear whether the buttplate and patch box ring are missing, or you are just describing them. Parts could be made, but working without originals to copy, a gunsmith would spend a lot of time and the cost could (IHMO) end up being more than the gun is worth.
The story about a sniper rifle is just that and adds nothing to the gun's value; it is very common to claim that almost any target rifle of the era was used as a sniper rifle and even that it was used to kill some prominent officer. Someone else may correct me, but I know of nothing that would indicate Wesson rifles were used as sniper rifles, and I would think shiny silver fittings would be quite undesireable for that use.
Again, I am willing to be corrected, but my WAG at value, in top condition, complete and functional, $1500-2000. Incomplete, maybe $500.
Jim
