A Farquharson actioned rifle set up for Long range target shooting as you describe will most certainly be a Gibbs Farquharson. The trade Farquharson did not come about until well after the day of the black powder long range matches. It will, as the others have suggested, likely be chambered for the .461 Gibbs. (which is undoubtedly the ugliest cartridge ever designed. <g>)
If original it will also likely have a Metford rifled barrel.
The two types of these rifles were the pure target rifles and the MBLs (Military Breech Loaders) The former tend to be finer finished. The later have a rear barrel sight as required for what were essentially military type rifle matches. Some rifles were set up for both disciplines. The earliest examples of these rifles had a side lever for cocking once in position. The underlever only brought the hammer to half cock. Probably a good idea when you are shooting whilst laying down...
As for value, the more original and more complete the set is, the more valuable. Such a rifle in fine condition with its full complement of sights (which will be in a case) would be worth a princely sum. If everything is junk except the action it would still be worth a couple $K.
Do post pictures.
Glenn Fewless
