Nice rifle, David, I've run a lot of pellets through a Shadow 1K over the last couple of years. Yours is a later model than what I've shot, but I don't think that muzzle brake is stock Gamo, looks too big. It also looks like metal, is it? All the Gamo brakes are plastic. If it's anything like the Shadow I shot, you'd do well to stock up on RWS Superdomes and JSB Exacts. Some do very well with CHL's also. 20 yards is a good range to start with, get comfortable there then stretch it out. Good luck, and keep us posted.
Dave