What type of bolt are you using? A level 10 or level 7?
Typically a leak out the front that stops when you hold the trigger is caused by a bad or dirty powertube oring or a powertube spacer that is too long if you are using a level 7 bolt. If you are using a level 10 bolt, the leak out the front may linger for a few seconds if the level 10 bolt is not perfectly setup. If the leak out the front does not stop when you hold the trigger then the problem is usually a bad or dirty on-off top oring, usually the small one.
Except for the Automag in front, its usually the man behind the equipment that counts.