Thanks everyone. I too am leaning towards dirty chambers from 38 loads. I still have 44rds of this Blue Dot load, so when I get back to the range I'll fire them FIRST, from a totally clean gun.
Kinda funny how no matter how experienced you are at reloading, things like this can still crop up sometimes.
TRE
I Agree that your problem is most likely caused by firing 38 Special rounds in 357 Magnum cylinder charge holes.
When shooting with a friend recently, he was unable to load 357 cartridges in his Model 66 357 Mag cylinder due to 38 Special residue in the charge holes. The fix was a brass brush and bore cleaner.
I know I'm in the minority but until recently I had never fired a 38 Special case in a 357 cylinder and have never fired a 44 Special case in a 44 Mag cylinder in any of my revolvers. (I do build 38 and 44 Special velocity loads in the longer cases.)
For the first and only time, for a test, 38 Special rounds were fired in my Model 66 357 Magnum cylinder.
The test went like this;
1. Visually inspected all 6 charge holes verifying all were clean and had no visible rings, marks or imperfections.
2. 24 rounds of 38 Special 158gr 825 fps rounds were fired in the 357 cylinder.
3. The cylinder was removed and the charge holes visually inspected. To no one's surprise there was a powder reside ring in each one.
4. Each charge hole was thoroughly cleaned and inspected. A faint ring was noted after firing only 4 rounds of 38 Special
in each charge hole.
5. The charged holes were re-cleaned letting Hoppe's Bore Cleaner soak for two hours. The faint rings remain in all charge holes.
Over the years I've run across a couple of nice S&W 357s that required a punch and small hammer to eject a factory 357 casings due to charge hole gas erosion from 38 Special loads.
We may differ on this subject but that's my story and I'm stickin' to it.