[1] Ball detents are one of the mechanisms often used in folding knives to prevent unwanted opening of the blade when carrying.
Some designs are similar to those outlined above, with a cartridge utilizing a ball bearing in a bore with spring pressure.
The cartridge is installed perpendicular to the barrel bore axis, just ahead of where the ball rests before being fired.
They rely on simple constriction of the bore to prevent paintballs from rolling through them from the force of gravity.
When the marker is fired, the air pressure pushes the ball through the bore, causing it to compress enough to pass through.