Thanks, I will try to keep the thread updated. The next major update should be range results from first batch of test loads.
I considered rechambering the barrel to 7.62x40WT but the tradeoffs in case prep time versus powder space versus bullet selection just lead me to deciding that for most of the applications for this rifle, a 125gr bullet is more than big enough to do the work needed and then I'm personally tired of dealing with case prep for the 300BLK. trimming 10mm off and reforming are few more steps than just reforming... the 7.62x40WT requires the same amount of prep time but just cutting 5mm off from the parent case (223 Rem).
I had a fun time shooting this upper in 300BLK at a 100 yard match using 147gr FMJs. I'll test these same FMJs later to see if they work with this new cartridge.