Coning a bore is tapering the bore back at the muzzle to make it easier to load the rifle or pistol, and that usually means loading without a short starter. (still depends on how tight a ball/patch combo you use)
What it boils down to is some like it, some don't, yu will always have dif. of opinions on it. My jest of it is the match shooters don't like it as they feel it will give them less accuracy. The hunters love it as they are shooting MID (minute of deer) and it is not an issue & worth doing because of the ease of loading & thumb starting the ball..
Personally, I think the only way to really know is to have a rifle with known accuracy with a known load, cone same barrel & then work up the best load for it with the coned bore, and then compare the targets........ Now to make this an fairly accurate test, one would have to do Several rifles same way, not just one rifle & it is a for sure thing...... I have not done it & don't know anyone that has on Lots of barrels. About 2/3 of the rifles I build have coned muzzles requested, and also 2/3 of my rifles are for hunting.
If I can shoot a 1" to 1.5" group at 50 yards with a coned muzzle, I am completely satisfied. My eyesight at long distances is long gone so I enjoy what I can of it for the distance I can. Bout 20 years from now it will be real easy to get the targets tho, as they will only be 2' away so I can see the bull !! ha ha ha ! :rotf: :thumbsup: