My Shilen barreled 340 is 26" long. Between 28 and 29 is where most traditional slow powders run with 250s. I have seen 2990s with 7828 with 250s using a Speer recommended charge (which is really tough to get into the case - lots of vibrating and/or a long drop.)

I don't recall the speeds on the 225s offhand but they were proportionally greater generally.

Sometimes, the air you 'let in'matters less than the air you 'let out'.