Harrows Opus Darts – Steel Tip – 90% Tungsten – 21g. Harrows Greatest Ever Creation – The Pinnacle of Dart Innovation. Manufactured in England, Opus features our new expertly milled acute facet cuts providing players with the perfect combination of grip and easy release. Opus darts are supplied with our new Slimcase, 2 sets of Carbon ST shafts, 2 sets of 100 micron Opus flights and are match weighed to +/- 0.05g.