There is nothing wrong with HS6 for snubby loads.
Speer Manual has "short barrel loads"
What brand of chronograph are you using??
10 to 15 feet is needed for distance.
The main issue I see is all the failures are due to KIMBER!
Yea old saying, the powders that do best in long barrels(velocity) also do so in short barrels. Which are slower powders. Unique can almost do it sometimes with some bullets and the Moon phase is correct.
No, I am not going to get in that old debate again.
My best guess is you have chrono issues
Don't know if you have checked out this thread?
Duplicating Buffalo Bore FBI Load