A lot of answers will steer you in the same direction, but you gotta arrive at the recipe that works best for you. Identify and focus on the points that have the greatest influence on accuracy; fore end hand positions, firmness of fore end grip, cheek position, cheek contact pressure (lots, little, none), firmness of butt against shoulder, etc., then experiment. Absolutely resting the fore end on a firm surface will kill accuracy just as an inconsistent hold and a lot of "twang" (spring vibration) will. It's generally accepted that a firm grip is a no-no with a spring gun but I had a Beeman Kodiak .25 that had to be held very firmly to get good groups. A good starting point for anyone with a "magnum" spring gun is an aftermarket spring/guide/seal kit that fit together properly to minimize twang. Did you buy your gun new? The 350 is a powerful gun I believe should shoot 8.4's faster than 888 fps. Have you checked a string to see if the MV is consistent and there isn't something else going on? Maybe some other owners can chime in. Also try shooting heavier pellets like JSB 10.34's to see how they group. Good luck.
