The balls may be manufactured from steel, hard alloy, titanium, aluminum, alloys such as brass or bronze, glass or porcelain, or synthetic plastics material such as polytetrafluoroethylene (Teflon???), polypropylene (Hostalen PP ???), polyhexamethylen adipamide (Polyamide A) or an acetal copolymer (Hostaform C) and materials having similar properties.