I have both books on sniping in WW1.."Sniping in French" and " a rifleman went to war". Interesting books, whch for me tell that even back then they knew a whole lot about a snipers Jobs/problems envolved on the battlefield, which is exactly the same today. A good long shot at 600-700yards was considered top class shooting with the guns they had back then.
Ofcouse as a owner of a .333Jeffery gun, I find it with a great interest, that it was the best caliber to be used against the german steelplates. I believe they must have been using th 250grain FMJ from Kynoch.
For the moment, I am scouting for books or info regarding the sharpshootings in the US civilwar.
I read somewhere that a unioncompany has some special muzzleloading rifles with special long barrels, that were used.I think it was in a Gun Digest I read it.