The old Speer "Atomic" #8 manual has 4227 at a 15gr MAXIMUM with a 158gr lead bullet.

That bullet you are using might be just a little fat, for max. loads?
You might want to check the dia. out, again.

I tried IMR4227 but found that other powders were cleaner, burnt w/o flake deposits and did not lock up my cylinders with light target loads that were too low for a good burn in a 686, 6".