I design Butterfly Swords (“BFS”) for personal use and on a professional basis (in the evening, not my day job) under the Modell Design LLC umbrella, http://www.modelldesignllc.com website. You can also see work product at http://www.everythingwingchun.com/wing-chun-butterfly-swords-s/35.htm web-site (look for the integral knives and flagship line). I recommend that anyone who has a serious interest in BFS read the following two articles:
– Jeffrey Modell, Esq., “History & Design of Butterfly Swords”, Kung Fu Tai Chi Magazine (March/April 2010)
– Jeffrey Modell, Esq., “Modern Butterfly Swords”, Kung Fu Tai Chi Magazine (July/August 2011)
BFS were actually used for the village water wars, by militia, security guards and criminal gangs. You can design them for cheap manufacturing or you can design and spec them as real weapons. I only do the latter but once you have weapons-grade design, balance, construction and materials, the knives are dangerous even if blunt.