Wad

A plastic or fiber support component placed between the powder and the shot to protect the shot at ignition and propel it uniformly down the barrel
Wads are one-piece columns with a shot cup, collapsible mid-section and an "over-powder" or "obturating" cup
It also:
1) helps contain the shot
2) seals off the shot payload from the ignition gases to prevent deformation and
3) protects the barrel from lead build-up
There are countless designs for wads, most designed to accommodate desired payload and case characteristic

Alliant Powder ®