Velocity and powder conundrum

[Harpoon1]

Hodgdon shows 28.7gr for max load of H335 on their reloading data page. That's with a Sierra 123 gr bullet.

So Hodgdon uses a 24” 1/9 twist barrel and the Hornady data is out of 18” 1/8 barrel.

If you subtract 150 fps, for the differences in barrel length. Say around 25 fps per inch! You’re basically only 50 FPS apart for additional 2.3 grains of powder! Something’s off, somewhere!

The only other thing I can see, the SST has cannelure and the Sierra doesn’t. So one is crimped, should have a lot higher neck tension than the other that isn’t!

Edited: just checked the Sierra data and they show 28.2 grns/2490 fps as their max, with 123 BTHP and 24” 1/8.