I just purchased a Marushin GBB M1 Garand and I am using green gas on it but the range on it is only about 5-10ft. I have tried to adjust it and I got it to the point where the BB would shoot almost straight up out of the barrel, and any deviation from that point on the hopup makes the BB shoot extremely short. The hopup is set with a tiny set screw above the bolt slide and that also might be part of the problem, becuase the blow back of the bolt is extremely hard and shakes most of the screws loose on the inside of the gun, the hopup set screw and a few others are the only screws I havn't put lock tight on. I  was wondering if anybody on these forums has ever owned or has one currently and can give me some advice on adjusting the hopup.