Don, I have pretty much found that whatever gun or barrel likes JSB's or Air Arms, they generally don't like anything else. Almost all of these are Lothar Walther barrels. I'm going to aquire some Crosman Marauder barrels, and test them until I find one that likes the Sniper Mediums. The barrel I now have on my Marauder likes them, but the barrel is damaged, and thus, fouls within 2-400 shots.
Check out some posts I had a while back, I describe my entire pellet sorting process in detail. I have also added even another step in the mix recently, by tumbling pellets in a rock polisher in order to smooth them out a little more. I do not use any kind of medium to Polish them, just use the action of pellets rubbing against pellets. Scott will tell you, as others will, I am a slight lunatic when it comes to sorting out pellets for big matches.