Though not exactly what you're hunting for, with mild to medium cast loads, you can't beat Unique. Going up the scale, either H110 or either of the 4227's fills the bill for me, or as the man advised, 2400. From your explanation, I can't tell if you lowered the muzzle between the two shots with the large variance. With 231, you're using a relatively quick powder with a low volume charge. Given those conditions, powder position is more critical than with slower powders. 150 f/s does not seem out the realm of possible spread to me. Could be the crony, but... What was the average velocity of the other loads? By the way, what was the charge you were using?