The compressed-air nail firing tool with head valve operates with single firing or repeat firing through valves comprising a selector, a distributor, and a shunt valve that can be operated through a trigger. The selector has two positions that can be selected from the outside by the user...http://www.google.ca/patents/US5671880?utm_source=gb-gplus-sharePatent US5671880 - Compressed-air nail firing tool with head valve, operating with single and repeat firing 
