That should not be a top end load. I'd be tempted to pop the top on the other jug and load a few using the new powder. From here, I'd think it's a powder issue. Perhaps it turned on you?

That was my first thought, that it had turned on me, especially since I just loaded about 200 rounds of 44 mag with it using 20.0 gr. Pulled out a couple rounds of the fresh 44 loads and shot them in a Rossi 92, where they showed no signs of high pressures at all. Since there is very little of the old powder left, will dump it and put some of the fresher powder in the measure then load a few more rounds and try again.

I know I have set the powder drop up thinking of load x and got distracted and had it dropping a different charge. Seems the most likely sinerio, pull a bulletin a remaining round and make sure of your charge weight.

Since these are the first 357 mags I loaded this go 'round, I fiddled with the measure quite a bit to get it throwing correctly. My press is an old Dillon 450 with the original measure that is a bit temperamental to adjust accurately, so each of the first few rounds gets weighed, than each 10 until I'm satisfied it is throwing properly. None of those measured from these 12 rounds varied by more than +/- 0.1 gr.

Keep in mind that 2400 was reformulated at some point. Mr Taffin reccomends dropping to 21 grains of the reformulated 2400 for the "Keith Load" in 44 Magnum. I'd probably drop my charge by at least .5 grain when I tried the new powder.

The powder used was from an old square can, don't know when they discontinued those, but it was probably at least 40 years old. I am pretty sure my 'new' container, at about 6/7 years old, is older than the reformulation as well, but I agree it would be prudent to drop it back a bit and work my way up carefully.

2400 was changed in the very early 1980's to the current somewhat "hotter" version.