The first question is what "conical"? "Conical" covers so much territory its hard to tell what twist he needs. People tend to lump all ML bullets as conicals and many are NOT conicals.
To get a decent read on the twist you need a tight patch and a rod with a ball or roller bearing handle.
If the bullet is short enough then a fairly slow twist is OK. I have shot maxiballs years ago with a 72 twist and they shop better than they should have but not good enough. Some 45-50 caliber "Express" cartridge guns had some surprisingly slow twists. 40 to 60". But shot short bullets. The 50 Ex. Winchester used a 300 gr in 50 is very short. Also accuracy was poor past 150 yards.
Then the "problems"...
Shooting elongated bullets in MLs causes a host of problems and this is the primary reason that very early in the development of these things they were found to be IMPRACTICAL for hunting use. Several reasons for this. One does not want to hunt and shoot game animals with a slow twist and a conical. They tend to veer wildly off track. This has been documented since the Crimean War. One does not want to carry a "naked" bullet loaded ML muzzle down for any length of time. There was a reason there was never and issued Minie Ball carbine for cavalry in the US military they would unload themselves unless carried muzzle up and this was not done horseback (this was chronic with the SB carbines as well). The patched versions were better at staying on the powder but require a lot of extra, heavy, equipment if accuracy is expected. If you doubt this you could make up a mould or swage for a "Pickett bullet" and try shooting it without a guide starter. I didas an experiment some years back. I then made a guide starter and got fair accuracy out of it. But at 200 yards a good RB gun in 50-54 would shoot as well if not better than the 48" twist 40 cal with a short FP bullet patched with cloth. AND for decent accuracy the 40 cal Picket needed 80 gr of more of FF. This is pretty common from what I have been told. The RB load for a rifle is about 3/5th the powder needed for the picket.
What your friend needs to do is look at the rifling depth. If it is over .006" and the twist is 48 then just shoot a patched RB. Its easier, its easier on the equipment and its much lower pressure and so safer.
Remember that the "conical" was shown to be a failure when used for heavy game in Africa and India in the ML era. Yes its documented. They used round balls for all heavy game. I.E. Such as elephant, Rhino, Cape Buffalo, Water Buffalo etc. So for the most part in Africa and India everyone hunted with a hardened RB.n Why people think the "conical" is superior for something as small as a WT deer never ceases to amaze me. If a 10 gauge hardened RB with 6 drams of BP will reliably kill an 7000-10000 pound African Elephant or the slightly smaller Rhino with lung shots why does anyone need a conical for a deer weighing 150 to 300 pounds?
The flat base shot much better. If I were to do this again I would want a twist around 30-36". But I consider is something fun to play with but its not practical.
If the barrel he has will not shoot with a RB then my advice is to rebarrel it with something that will.
