Why shorten?
Because he is very low on pressure because of the over length suggested by Hornady
I know there's a couple schools of thought here, and no answer is right or wrong, but . . .
I'm sticking with my thought process: If the rounds fit in the mag, and the slide goes into battery, I don't see a reason to shorten the OAL.
Low pressure certainly isn't a good enough reason. Increase charge or move to a faster propellant.
The the OAL "suggested" by Hornady isn't really a suggestion. It is simply what they used. I agree that Hornady is probably loading to an OAL that'll work reliably in just about any firearm, and that's a good thing. And when in doubt, it's probably a good way to go. I agree. But it would seem unnecessary for our new loader to shorten the OAL. He certainly may, but I don't think it's necessary.