Solid cleaning wads. Their diameters are such that they are a tight fit in the bore. When the weapon is fired, the pressure in the barrel and the acceleration of the felt pellet cause it to shrink in length. This lengthways shrinkage causes the felt pellet to try and expand in diameter. This increases its pressure on the wall of the bore, giving rise to the desired high friction. The felt pellet presses itself into the rifling and follows its twist. This ensures that every part of the bore is reliably cleaned.