One more idea - Maybe the ID o-ring in the hammer is worn and allowing too much gas to bypass the hammer.
Hi UJ, Hector...How low is the velocity? I've seen numbers from 350 - 375 fps with 13 - 14.3 gr pellets (for oem stock trim).If it's lower than that then...Are you getting much more than 30 shots per cart? If so, you may have a weak hammer strike.Possible causes:A bent cam that is hard to rotateA cam ring that doesn't slide smoothly along the camA feed arm that is binding on the frame (sometimes on the bottom of the feed arm)A hammer that doesn't slide smoothly in the pressure tubeA hammer sear that isn't smooth and drags too much on the hammer.A sticky exhaust valveThere no doubt is more...If your shot count is normal - i.e. 30-35 shots per cart, then maybe you have too much gas blow-by in the firing cyclePossible causes:A damaged breech seal (they require some finesse to assemble without damage)A large gap between the feed arm and the barrel ( should be about .002 to .003")A worn or damaged barrel.There is no doubt more here as well.There's a lot going on with the action in the 600... a lot that can go wrong.And lastly, make sure the setscrews that secure the feed arm to the cam are not loose... they like to get loose. Check them often. hth-Wyo
Update: Partially disassembled the gun and everything seems to be in order.Shooting it today we discovered that it performed well when only one pellet was loaded. Loading three or four pellets the power was low, except for the last shot which was noticeably louder (we didn't have time to check velocity with chrony). We tried this several times with the same results. Any ideas as to what in the cycling operation could cause the power to drop off when loaded with more than one pellet?We are stumped but not beaten.