The designated mold size indicates the size the balls should drop from the mold, not the exact size of the mold cavity, so shrinkage is already allowed for. Still, I would go with .380". If cast of pure lead you'll have no trouble seating oversize balls and if you ever decide to get your chambers reamed to full groove diameter of the bore your .380" ball will still be tight, whereas a .375 may not be. I load Hornady .457" balls in my 1860 with chambers reamed to .452" and seating them is no trouble.