Entertaining but really doesn't establish anything
I've experienced greater variations between two 7.5" barrelled SRHs due to differences in cylinder gap, cylinder throat diameter and bore size. If the idea is to establish effect of barrel length on velocity for a given cartridge you need to start with something like a universal receiver fixture, and a single barrel. After the first shot string, remove barrel and turn off an inch, shoot next string, repeat. Anything else and we're just kidding ourselves.