I do have a suggestion for the valve retaining screw. This works very well in conjunction with the HiPac, and I got the idea from a post by airgunsnirvanna here I believe.
Replace the back half of the co2 valve with the back half of a Disco valve. Now by slightly enlarging the valve retention screw hole in the tube, you can use one of the Disco valve retention screws to "pin" the valve. Works like a charm!
hope this helps