Hi Iain,
Can you pull the top of the action back with the other hand as you fire it? I am just wondering if is binding slightly?
Did the front two pins just glide back into place on re-assembly? Sometimes they need to be swapped over. The tolerances are very tight. Did you put any lubrication on them and what sort did you use?
My guide is here with pictures if it helps.
http://www.airgunbbs.com/showthread....bau-Model-300s
HTH
Mark