Many use standard cast bullets for use in sabots, but accuracy will be much easier to obtain with bullets having smooth sides, so the sabot has a better contact area. I make them with a small bevel so they set down tightly in the sabot, with whatever weight the customer wants. For best accuracy at maximum weight, the bullets bearing surface should not be longer than what the sabot pocket will cover. When ordering a mold, I like to have the customer send a sample sabot that he wants to use and let me make the bullet to fill it, if he wants maximum weight. If a lighter bullet is wanted I fit cut the bullet to settle down neatly into the sabot and at the weight desired. Hard cast will give as good accuracy as the best jacketed bullets, with far better killing performance. (You'll probably find that jacketed are considerably more accurate than grooved cast bullets.)