When you have your barrel chambered the smith can play with the starting depth of your rifling. You can have your bullet jammed into the rifling when you close the bolt, or you can have the barrel free bored like a weatherby. You can have the smith start the rifling to provide a very short jump for your bullets ogive into the rifling at your prefered seating depth. I have shot 129 Hsp and the sst's. The SST's did not shoot well for me. The 129 interlocks shoot very nicely in my rifle along with the 125 Partition and the 120 NBT. I have a 1/8 twist so the longer SST should not have been the problem. The 129 SP load is with 48 grains of R19, Fed 210 primers in preped Rem brass. My empties show pressure, but I do have a short throat.

It seems to me that the ONLY reason for any change in chambering is so you can play with a different cartridge. Be honest with yourself, you are a cartridge junkie. Fortunately for you we have a meeting here 24/7 but you do need to stand up and admit your addiction. Really, things will be easier once you've done that.

If your 260 is on a med action you may have problems going to the 6.5/284. Should have a long action and most 6.5/284 are set up for the 140/142gr bullet and the velocity is around 3000fps. Most of the AI I've had done the velocity gain has been 100/150fps more is it worth it? I like the AI case in fact I'm having a 22-250AI and 243AI done up right now. I think the 260AI would be a good choice and get it twisted for the bullets you want to shoot my 6.5/284 is set up for the 140/142gr bullets. I shot a 6.5/55 for afew years and kind of wanted to do that in a AI and from what I hear it's a pretty good round also and would work in your action. Well good luck, Tom