Larry;
Good questions all, you are starting to sound like you may actually be interested in learning a bit. There are in my opinion a couple of key points involved here. One you have already mentioned is that DuPont's tests were all made with loads having the same terminal ballistics, but with widely differing burn rates of powders.
Bell's tests on the other hand, as you also mentioned were made with loads of differing terminal ballistics & with a far lesser variation of burning speeds. I looked up Universal Clays on Hodgdon's burn rate chart & only found Universal without the Clays attached. I do not know if this is exactly the same powder or not.
The burn rate for Universal falls between Unique & Herco. in comparison to Alliant powders so is just a bit faster than 7625. Black powder burn rate is close to the 7625 so it was, in fact, a slow burn powder. The early smokeless powders were all faster than black so gave higher max chamber pressures than did Black, but lower barrel pressures. I believe that DuPont Oval was the first of the so-called progressive powders. When Hercules brought out Herco it was "I Think" just a tad slower than Oval.
As you can see thus Bell's test were all done with powders of very close to the same burning rates but with different terminal ballistics. so, unfortunately, do not provide much help as far as this discussion goes.
Incidentally, the Infallible powder used in the DuPont tests had the exact same chemical composition as Unique. Both were flake powders, but the Unique flakes were about 16% thicker than the Infallible flakes so just a WEE bit slower burning. In these DuPont tests, Black fell between Infallible & Oval. Ballistite & DuPont Bulk Shotgun smokeless was just a shade faster than Infallible but were used in similar loads. Oval was designed specifically for the new Higher Velocity loads with the maximum shot weight, which for 2 3/4" 12 gauge was 1� oz. The early 3" 12 gauge Magnum carried 1 3/8 oz & the so-called Super 10 (2 7/8") 1 5/8 oz. DuPont specifically stated the 3-dram equivalent load of Oval, even with the full amount of 1� oz shot was an "Underload".
My loading has always been aimed toward hunting loads so a powder which works well at any temperature encountered is desirable. With this in mind, I believe one will have far better results to stick with powders designed for the purpose. For light shot charges thus a powder in the fast range is desirable. For heavy charges at maximum velocity then a powder in the slow range is called for. Medium powders are for the intermediate ranges.
For strictly warm to hot weather conditions, a bit more leeway is available, but I still see no need what-so-ever to go to those extreme low pressures by taking a slow burn rate powder out of its design element, even though some loading manuals list such loads.
I will note that unless it has occurred in the very late years Hercules/Alliant has not listed any extreme low-pressure loads with their slow powders, For Good Reason, I believe.
