I have been reloading 357 mag for years, but my latest range session turned up something strange. My brass is VERY dirty after firing. 18.2 gr 300MP, 158 gr JSP over small pistol primer. I was shooting them out of a Rossi 92 20" barrel. I had never noticed my brass getting this dirty, but then again, I have only had the lever gun for a short time. Is there something different about the way this cartridge reacts in a lever gun as opposed to a revolver that would make it so much dirtier? Is this even something that I should worry about?

Might need to bump the charge up a bit.

18.6 gr is max. I'm already very close to max.

You're powder charge is fine. Are you running a mag primer or standard? I'd run a mag primer with this slower powder. This could help. It's not uncommon to get a soot around the brass case in a straight walled round. Just keep shooting and cleaning your brass!

300MP in 45 Colt Marlin and Rugers was miserable with mag primers, unburned powder and lots of soot. I switched to standard primers and bumped the powder charge and they cleaned up. I have not tried Mag primers with the 357, works fine with standard primers.

Are you running pistol reload data or rifle reload data for that round? The max loads for pistol and rifle are different in a couple of the manuals that I have. I'll be doing the difference testing in my 44mag carbine and pistol in the winter

I am running standard primers, not Magnum. I got my load data from alliant, and they said to use the standard primer, not mag. I am running the load through both my blackhawk and rifle. Because I want to have one round for both firearms, I worked up the load in the pistol. I have not noticed the same amount of soot when fired out of the pistol, and this is by far the most accurate round I have ever loaded in the pistol.

How tight a crimp are you running, a little more crimp might help.

I believe my crimp is tight enough. I have run a heavier crimp in the past, and had issues because my crimp was too tight. I might try a slightly heavier crimp.

Over crimping can ruin neck tension. What brand are the dies?

Lee dies