I tried the 129 gr SST in a 260 Rem I had a while ago. I never could get the SST's to shoot as well as the A-Max (120 and 123gr versions). I couldn't find the A-max anywhere when I shot up what A-maxes I had so I switched to the 123 Scenar and haven't looked back. I sold the 260 and am currently loading the Scenars in a 256 Newton and I'm planning on giving them a shot (might be a pun there) in my new creedmoor.
