If your gun is a fast twist shallow grooved rifle designed to shoot sabot style projectiles it will not shoot the hollow based full lead "minne" with any accuracy what so ever.
The design of the projectiles are completely different!
The minne is slightly undersized for quick loading,and its "skirt" was/is for expantion to grasp the rifling of the gun using powders with a slower burning rate or more fouling than the pyrodex P(pistol) grade than your using. When using the minne for maximum accuracy, finding a proper lubrication is/was always the key variable.
Sure,,you might see a high or proper velocity across the chrony,,but if that long full lead projectile "skip's" the rifling,,,it won't spin proper for accuracy.
360gr in a .50? That's a long heavy bullet. yes. they make'm.But it's up to you to learn how to make it spin,,,the minne was made for fast loading,easy cleaning, breaking bones and slappin big holes in meat up close,Civil War stuff,the guy was French, his name was "Minnie"