You are absolutely correct, you have an excellent memory. I tried everything, way back when IPSC/USPSA was young, to find the best powder that would take my 38 Super to major in a commander length barrel. No one else back then was shooting a short racegun, so it was new territory. Takes a slow powder, which both 296 and 571 are. 571 is more traditionally a mag shotgun powder than a pistol powder. I believe when Winchester stopped making it that it was replaced by the identical HS-7. I never bought any HS-7 as I bought up a lot of cheap cases of 571.
When I was looking for the proper powder to accomplish my task, I tried stuff like 296 and H110. They were too bulky in the small Super case. I don't know how many different powders I tried, it was a bunch and I saw a lot of flattened primers and excess pressure signs as I'd ease my way up the load chart. Pulled a lot of bullets when lower powder charges showed signs I was exceeding safe pressures. Finally though, 571 did the ticket. And one of the essentials is that it is easy to measure and stay in a narrow weight variation since it was running way up in the pressure and we were cranking out thousands of rounds at every sitting.
Understand also, that in the 80's, major power factor was 175 and we had to exceed that. In the 90's that got dropped to 165 which is a lot easier and opens up a whole lot of other powders. I always and only shot 115 gr JHP out of my compensated racegun. My target velocity was 1550 fps. Compare that to 357 magnum sometime to get an idea how much oomph we're talking out of what was basically a thicker walled 9mm case.
Typical months workout was usually around 3-4k shots fired in the city owned indoor shooting range we, as a club, had access to at all hours. So I'd shoot 10 to midnight when no one else was around, several night a week. And dry fire practice at home the other nights. While I had the property to shoot on, when practicing neighbors would almost always call the cops that 'automatic weapon fire' was occurring up over the hill, and we'd get a visit from the sheriff. It was easier to just take the practice to the range where no one noticed.
The gun I hand built is still my fav. It has over 135,000 rounds through it. It is also one of a kind.
Warning - Graphic gun porn image -