Some time back I purchased a 2nd hand AA S410. Never had any problems filling it. I decided recently to re-do the stock, emptied the cylinder, finished off the stock this afternoon, and put the package back together. When I tried filling up the cylinder, it seemed to push any air going in straight out of the barrel, without the gauge moving an inch of the zero mark. Also, what is the purpose of the large grub-screw inside the trigger guard? Please help!