I pretty much have to agree with what FN said. My 350 .22 seems to prefer the lighter weight pellets, 14.5 to 16.5 gr. Best accuracy and power, especially with Predators at 16.5 gr. It also seems to prefer longer pellets, I get the most power from FTS's, but it throws them all over the place. Nearly all my chrony numbers were higher than SS's also. I have a nearly new Chrony Beta, I set it up 3 ft. from the muzzle.
Mine doesn't seem loud to me, either. Even shooting in my shop, I'd have to say the pellet hitting the back of my trap is noisier than the gun. Trap is home made, but the back is an angled peice of 3/8" aluminum screwed to 1 1/8" concrete form plywood. Lots of long screws with rubber washers under them, so it doesn't ring, just SMACK! Weighs a ton, but it does the job. Never had a pellet go supersonic, fastest so far has been FTS's at 867 fps. BTW, RWS, the importer, claims 1100 fps. Diana, the manufacturer, only claims 900. I think it will do it with the right pellet, maybe RWS Hobbies, or what's the Beeman Silver Bear weigh in .22? They're light weights in .177, only 7.1 gr, shot some in my CFX last night, avg. 940 fps. Now I need to try them for accuracy. HTH.
Dave