Easy guys! It is easy to see how one could be concerned with the velocities that are observed with a creed and rl26, I myself have a .260 AI with a 29" barrel, and I maxed out at 3070-3100 with that using rl26 and the 147, and currently run the 156 at 2940, max with my components is right about 3000. So when a cartridge with about 5 grains, nearly 10% less capacity, and a 3" shorter barrel is nipping on my heels, it really makes me wonder.
However, I know what it is.....I seat my 156's at 2.950" to feed in my AICS mag, and many with a creed are very close to that OAL with the same bullet. For the 156 to be optimal in my .260 AI case, my OAL would be about 3.090, that puts the bearing surface just up into the neck. That is .140" of powder space I am not able to use. Now if he is limited to 2.870", that means (since his case is very similar to mine) that there is .220", or nearly a quarter of an inch of powder space that is unusable with a 156! That will make all the difference in the world, and there will still be significant room taken up with a 140, and at 2.870", will make the useable powder capacity of the .260 AI/terminator nearly equal to that of a 6.5 creed, answering the concerning question.
To Stgraves260, if you get around to it, put a bainey box (3.110) in your rifle, seat your bullets out, and that 140 will be well over 3100 fps. I was getting 3140 from my 29" .260 AI, and I didn't have a +P throat, just 3" more barrel. These cartridges nearly need a medium length action with these long bullets!! When I burn my barrel out, I am going to put that box in mine, and go back to BDL set up.