Generally, the length of the bullet vs. bore diameter is going to control what is the BEST ROT for any given conical. If you shoot a conical in a barrel with a slow ROT designed for shooting RBs, the conicals may not be spun fast enough by the rifling to stabilize them, and they will yaw in flight, and "keyhole " on impact. Group size will be larger, and poor, and unpredictable. That is, you won't be able to be sure where the POI will be in relation to your point of aim( POA).
Generally, RB barrels will have a ROT that is 120 times the caliber. But that is just a general rule. Adjustments also have to be made for velocity. And, a RB can be fired from any ROT barrel- as proven with pistol barrels with very fast ROTs. The Slower the ROT for a RB, the more foregiving the barrel is for accurately placing balls on target with different powder charges at the same POI. So, while my 1:48 ROT .50 cal. barrel shoots RB accurately enough, It won't handle the very heaviest of powder charges sometimes used in heavy barreled rifles, with a slower rate of twist- say 1:66", or 1:70".
ROT for conicals generally begin at 1:34, and go faster from there. You see 1:32, 1:28, 1:24, and 1:22" ROTs. The faster the rate of twist, the longer( and therefore heavier) the bullet that rifle barrel will stabilize.
For example, the modern .458 Winchester Magnum Cartridge barrels have a 1:14" ROT, while the standard ROT for the .45-70 cartridge is 1:22, with target shooters and long range BP Cartridge silhouette shooters opting for a 1:18" ROT. Both cartridges shoot the same diameter bullets. Only the shape, and weight of the bullets used differ, depending on use of the rifle. BTW, the standard ROT for the .38 spl. revolver cartridge is also 1:14".
If you have a particular caliber in mind, let us know. We can tell you the optimum ROT for RB, and using the Greenhill formula, tell you the better ROT for a conical.
Now, as a word of caution, If you are using one of those modern zip guns, that we don't discuss here, that calls for using a plastic shoe, and a copper jacketed pistol bullet of some lesser caliber, all bets are off. You go with the factory recommendation, or do you own research. I have seen, and heard of too many shooters "sighting in" those "guns" where the best they do is to hit close to the 12 inch square paper target at 100 yds. They then leave the range, saying they ought to be able to get a deer at 50 yards with that kind of accuracy! :shocked2: :youcrazy: I don't want those shooters in the woods, but I have no control over that.
The best I can do is find out where they are hunting, and hunt somewhere else. :wink: