I've just done one funnily the other week, as with the 50 the secret is to wind in the spring compressor with a small socket in the end of the inner tube between the end of the compressor to allow the tube to turn, wind it in slowly a few turns at a time & use a flat screwdriver to align the two tubes as they want to turn apart as you go, when you get the pin holes in line pop the rear pin in first till you get the second one in then refit the trigger & end cap. make sure you get the outer tube in the right way too not upside down too .Oh & grease the 3 balls holes well to stick them in place before you start too. it's really a three handed job tbh so a competent mate to align the tubes as you wind in the compressor would be ideal. The one I did had the one of the tabs at one side the trigger pivots on bent out so i had to tap it back straight to get the trigger pivot pin in so check they are parallel before refitting the trigger pivot pin. An airsporter spring brings them up to power i found but a little harsher to shoot so i'll be fitting a weaker one soon .