I couldn't find my old thread and thought I would post an update as others might find it useful.
We loaded up .243 using this method with the lightest bullets we could find, 55 grain:
https://hodgdon.com/wp-content/uploads/2019/09/trail-boss-reduced-loads-2018-2.pdfTook them to the range and wow, those are some light loads for sure. I don't think they are supersonic and were much lighter than I expected. I didn't shoot them myself but judging from the reaction it looked closer to a 7.62 or 556 than .243 for sure.
Grouping was about 2" and pretty inconsistent but I'm guessing we could go through load development and get them tighter if we wanted to. I also didn't load them with much precision and used Wolf primers. We just wanted a couple boxes to get him started and then transition into full loads.
If you have a kid starting out or someone that likes shooting but not recoil, it's a great way to go. We didn't have much trouble finding Trail Boss, probably not the most popular powder out there.