RE: .50 Lee Minie 360gr.
Have you slugged your barrel to compare with the diameter of the ball you are casting? You might have A. the wrong sized mould, or B. the correct size mould by Lee standards that is casting undersized. Lee moulds from my experience are hit or miss, I recently bought a .324" rifle mould that makes .323" bullets- nearly worthless to me as is.
One trick I recently learned is to increase the size of the casting by applying a few strips of tape on the mating surfaces of the mould to open it up a few thousanths of an inch. It will give you a projectile that is slightly oblong in profile, with a rifle bullet, it doesn't matter much if its resized, but I'm not sure if it will work with a ML bullet. The tape I use is aluminum duct tape which is about .003" thick, but I've heard that thinner paper held in place with adhesive can also be made to work.