Correct, the shroud to the bullet in the rifle. Available here:

http://www.midsouthshooterssupply.com/item.asp?sku=00072H15045BR

The crushed rib have grooves in the side of the sabot, allowing the surface are of the outside of the sabot to squeeze in better on a tighter bore. They shoot very accurate for me in my Knight Disc (Origonal Disc) and my TC Omega using 300g Nosler Partition HG, this one:

http://www.midsouthshooterssupply.com/item.asp?sku=0011545350

Here is what the Crushed rib look like. Chap Gleason