First off here is the manual in pdf format so that you can familiarize yourself with your markers workings http://www.airgun.com/Images/rtmanual.pdf. Like others have posted this can usually traced back to a few relatively easy things to fix (1) a worn/modified sear (2) a worn/modified on/off pin (3) output pressure of adjustable tank too high (4) worn/damaged regulator pin also you might just for the heck of it buy a parts kit and replace all the Orings . If this doesnt fix your marker it may be time to consider sending it to airgun designs , tuna or one of the other techs that frequent these boards
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