Swifty, thanks for your correction, sorta.
The Hodgdon website does assert that W-540 and HS-6 are identical. They are virtually the same burn rate and have nearly the same GraviMetric Density (GMD). However, most of my use of the fast powders is for handguns, so I use powder measures rather than weighing each charge. Shotgun reloaders usually do the same according to my reloading friends. Although W-540 has been rated at the same burn rate as HS-6 they cannot be interchanged in any powder measure; they occupy different volumes per grain.
HS-7 GMD is shown in Lee's Reloading Manual, 2nd Ed., as 0.0680, same as W-571. That's why I said they are identical.
HS-6 " " " " " " " , " 0.07120. W-540 is shown as 0.06830.
According to Lee's data, W-540's GMD was bracketed by AA-2700 and AA-2520, W-571 by HS-7 and AA-2520, and HS-6 by H-1000 and RL-25; The Density/Volume ratio puts these pistol/shotgun powders in rifle territory, bracketed by slower powders.
Hodgdon's mention that these (HS-6 and W-540) are identical or interchangeable is probably based on their VERY similar burn rates. That claim maybe also falls into the poetic license category, since they are the company marketing Winchester (and IMR) powders now. I have not seen any mention of W-540 or W-571 powders in Hodgdon data recently.
Don't use the same setting on your powder measure for HS-6 as you used for W-540. They are "truly outstanding" but not interchangeable.
if the manufacturer claims "identical", I am sure there is no "poetic license" involved. I don't know how you would even consider that.
The differences you describe are simply explained through different lot numbers of powder. Every time you bake a cake using an "identical" recipe, do you get "identical" results?
