I believe ball seat is a military term. A hangover fromt he days bullets were all called balls. That is all guess on my part, but I will clarify exactly what a ball seat throat is.
It is a throating type with a straight cylindrical section larger than bullet diameter, but not necessarily as large as groove diameter. Some high grade rifles which give superb accuracy even though groove diameter is much larger than bullet diameter, have a close tolerance ball seat throat which removes only part of the rifling, and aligns the bullet perfectly. Some ball seats are cut way larger than standard bullet diameter, which is detrimental to accuracy. -- I prefer rifles with ball seats for cast bullets, as fitting bullets requires simply sizing to whatever will fit the ball seat closely.
The other type of rifle throating is tapered, and the length, angle etc varies dramatically from gun to gun. Very heavy usage equals a long taper throat, due to powder erosion. -- To get best results with this type I cut the bullet nose close as possible to groove diameter, or the point in the throat that it will be when chambered. The bullet body is sized to fill the case neck to a close fit to the chamber, with a bearing length to come close to the throat taper. If you find all this confusing, don't be concerned about ordering a mold. Just send a throat slug with your order and I do all the figguring and cut the mold to fit.
