" I've worked with strain guage equipement and seen the curves so I'm at least wet behind the ears on this one
... I'm talking about barrels that are short enough in a given caliber that you CAN NOT burn a full charge completely."
I am too; it doesn't matter where on the optimum pressure curve the bullet exits nor if all the powder has burned, which is rare for any length of barrel. A bullet has no idea of how much powder remains unburned anyway, it just runs down the barrel until it's suddenly free! I know the powder burn facts and am simply responding to the OP's rational question in view of the often suggested - including here - but incorrect web info of using a faster powder in a short barrel by explaining why the optimum speed rifle powder choice is irrelivant to barrel length.
I have no idea how wet your ears are, I never addressed velocity and care not what your 7STW muzzle pressure is because none of that is relivant to the question. Nor have I any personal interest in competing with you for being perceived as the resident site guru. Thus, you may happily post any opposing views you feel you need; I'll just laugh.
Aramarine6, your post count is fine and you should indeed do the experiments as you plan. BUT, you should recognise up front that what I said would require you to first test the long barrel for speed and then cut it to test the short version of the same barrel and chamber with the same load.
It's not uncommon for 'identical' factory hunting rifles to vary a couple hundred fps with the same load! Simply testing different powders in one length can only give you the results from that specific barrel, there's no way to compare the results to another rig and say the barrel length is what caused any differences you may see.