It's a polymer coating, proprietary according to their website (assume it isn't hi-tek since it isn't advertised as such). All feedback I could find on IMR 4227 pointed to full case or even compressed loads working best... contacted KC to see if this was advisable with their bullets & they replied that it would be just fine. Really no danger other than some leading, so off I went.
Started with Hodgdon's minimum for 158 grain bullets, and the first few charges were so-so. A little dirty, and mediocre accuracy. The magic started when I hit 15 grains, which if my eyeballs are correct, is also about where it hits 100% load density with this bullet when seated to the crimp groove. Cleaner burning, and very good accuracy. Went as high as 15.7 grains; all loads at 15.0 & up were good but saw diminishing returns higher than 15.2 so that was declared the winner.
Had zero leading in the barrel, attributing that as much to the correct sizing as to the coating. Bullets are a consistent .358, and my barrel slugged .357. The coating does leave some residue behind, but it's neither excessive nor difficult to remove (easier than dealing with copper).
Velocities were very consistent at 15.2 grains, at least with the small sample size of 5 rounds I had left after accuracy testing:
1501 fps
1515 fps
1502 fps
1512 fps
1505 fps
I don't expect it would stay that tight over a larger sample size, nor do I think a 357 Magnum rifle load intended for sub-100 yard shooting needs to be that tight... but I sure am ok with it!
