If it wont tighten to the point where you can only pull the trigger once, then you have a leak in the system. I suggest oiling all the o-rings and inspecting them. If that doesnt work you need to replace the o-rings. This is the cause of your problems unless you have multiple issues.

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'07 Proto Matrix Rail (all composite changed to aluminum)
Dye Ultra-Lite frame
Dye Throttle 70/45 HPA tank
Deadlywind Carbon-Fiber barrel w/ SS Freak insert kit
Halo-B V35 board, Rip-Drive, & Bone boost battery pack