My findings and meager experience with loading a coned muzzle on a gun made by one of the lads here, was that quite loose combinations (by my standards) could easily be loaded. Also, a tight load was almost impossible to load even with a short starter. The reason for this anomaly is pure physics and had to do with angles associated with "drawing" metals.
My non-coned muzzle, same calibre, loaded that combination easily, with starter & the rifle's 3/8"rod.