This is the Speer 150 grain 300 Blackout bullet load I settled on. As you note... I settled for less velocity for the accuracy, even though the faster loads were far from inaccurate. Probably couldn't do this again if I had to (note that it's only 50 yards);
Do you know what barrel length Speer is using for its figures?
16" Barrel.
Here's the first page with the details;