What is skirt quality and why does it matter?

The skirt is the thin, hollow rear of a diabolo pellet.

It must be slightly larger than bore size so the air pressure expands it (obturation) and seals the bore. A damaged, out-of-round, or too-small skirt blows by air, drops velocity, and tumbles the pellet. Inspect skirts visually before loading; flash, dents, or split skirts go in the discard pile, not the gun. Skirt quality is one of the biggest reasons premium pellets shoot tighter than budget pellets.