My shooting buddy and I are prolific handgun reloaders, shooters, and both members here and have grown to like the traits of IMR-4227 powder. I bought us each a pound of 4227 about 2-3 months ago as we were running low. I noticed this batch was made in Canada, older batches were manufactured in Australia. Anyway he just opened up this new bottle, loaded up a batch in 357 Mag brass with 15.0 grains behind one of his 158gr mold gas check bullets he made with a double crimp groove, crimped on the lowest groove giving the longest OAL possible. Actual weight of the bullets, lube and gas checks was 153gr. as it was a very hard alloy. He loaded up a cylinder full in a 4" 19-3 and shot 3 times, the third shot locked the gun up by so much rearward pressure of the brass against the recoil shield. After getting the cylinder open and having to drive the empty brass out we discovered the cylinder has three bulged chambers, all primers were flattened, one fell out of the brass upon opening.
In the past using 4227 I have compressed all bullet weights in 357 brass and never had an over pressure result. He and I both pulled bullets from this batch and found 15 grains which is not a max listed load. It'll be interesting reading your responses.