From Powerbelt:

PowerBelt™ Bullets are the most advanced muzzleloading projectiles ever developed - with all of the benefits of saboted bullets and none of the problems. Like sabots, the PowerBelt™ Bullets' patented snap-on base creates a perfect gas seal, providing consistent pressures and unsurpassed accuracy. But unlike sabots, PowerBelt™ Bullets are full caliber sized, easy to load and do not require cleaning after every shot.