There's a couple of major sabot manufacturers, and MMP make sabots having different outside diameters. Harvester sabots are ribbed. A variety of sabot types may need to be tested along with different bullets, powders and loads. 90 - 100 grains of powder usually works, but a little less could be a starting point from which to gradually increase the powder charge in 5-10 grain increments especially if shooting the more powerful 777 loose powder.
Shooting at a moderate distance like 50-70 yards helps for accuracy testing by reducing sighting/shooting errors.
The most popular bullets seem to weigh about 240-250 grains.
http://www.mmpsabots.com/
http://www.harvesterbullets.com/