Sorry, but barrel length vs. life doesn't add up. Most barrels burn out at the
throat not the muzzle. If your logic were correct, then why aren't the 30-06
(the parent cartridge of all listed) and the 270 not 26" also???
All longer barrels take advantage of slower burning powders and will maximize
velocity.
As to why H&R chose only those two calibers, I don't know.