I feel that lead purity makes a big difference. That's why I insist on only as pure a lead as possible when I shoot muzzleloader projectiles. Of course in the case of the polymer shooters ( a.k.a. the milk jug people ) the sabot protects the bullet from fowling up the barrel when pushing the projectile at the higher speeds I would think.

Sabotloader, did you shoot the conicals that came with the rifle? You must have had some fun with them when they thumped down range.