2400 is not a good powder choice to use in a 4" anything.
I'm curious what you mean by that 44 AMP? Or maybe a little more detail.
From my point of view, its a matter of performance and efficiency. In the shorter barrels (and I mean shorter than 6") 2400 usually gives around 100fps (if that) more velocity at max than some medium burn rate powders, and you're burning more powder to do it.
Say, for example, you load 12gr of 2400 and get mid 1200fps and you load 8gr of Unique and get high 1100s (numbers for illustration not actual specific loads) you're burning 50% more powder for a very small increase, and one that might not even exist in a different gun.
Variation of even 100fps can happen between different guns with the same barrel length, shooting the same ammo. Its not common, but I've seen it happen.
When the barrels get longer the increase you get from 2400 far surpasses the small lead 2400 can have in shorter barrels, which to me justifies the greater amount of powder burnt.
The gun matters A LOT as well.
When the stars line up some unexpected things can happen. Back in the late 80s a friend came over with some "hot" `125gr JHP loaded for his Marlin carbine. The powder was 2400 and the amount was a listed load in the Speer Manual of the day. (and way above the loads listed in the OP)
Since I had gotten a chronograph he wanted to see what they actually did. And since we were going to test, we added in the other .357s we had available at the time. A Marlin 1894 carbine, a S&W m19 6", a S&W M28 6" and a Desert Eagle (6").
First gun tested was the M19. The guy shooting it doubled the gun! (fired two shots when one was intended) To this day, I don't know how he did it, but it happened. The one shot the chrony picked up read 1620fps.
We decided that was enough for the M19, and while the 4 unfired rounds fell out normally the fired cases had to be driven out with a rod and a small mallet, hand pressure on the ejector rod would not move them!
Same ammo fired 6 normally through the M28, and after a slight resistance, ejected normally with hand pressure alone. Avg vel was 1670fps!!
Same ammo fired through the Desert Eagle, one full magazine (9) function was flawless and the velocity was 1720fps!! And while the DE was nominally a 6" barrel that length also included the chamber. In addition to no flash gap the barrel was also polygonal rifling.
SO, 100fps difference in MV between 3 different "6 inch" barrel pistols. Uncommon, but not unheard of.
Same ammo fired from the Marlin carbine clocked right on 2200fps.
This story isn't meant to tell you what will happen, or what usually happens only that sometimes things like this DO happen.
I no longer have any .357 with a 4" or less barrel, all mine are 6" or longer and for them, 2400 is a great choice.
were I loading .357 for shorter barrel guns I'd load a faster powder than 2400, expecting to get about as much, and waste less powder. But, that's just me, and if you get results with 2400 that you're happy with, in a 4" by all means, use it.
Now, I keep hearing how today the powders with the old names are not the same as they used to be, and only modern data should be used with modern powder, and I won't dispute that. It's never wrong to be safe.
However, personally, I still have the powder I used back then, and still load the loads I worked up back then, and they are still the same in my guns as they were back then. When I do finally get new production 2400 I will work up my loads with it all over again, just to be safe.
One more point, about 125gr JHP and carbine speeds. You need to CAREFULLY choose the bullet for proper performance. Nearly all the 125JHPs are designed to open up at handgun speeds and the extra 500fps + you can get from the carbine overspeeds those bullets beyond their design limits and they expand explosively when they hit. (think varmint bullet expansion) This usually results in very poor penetration and therefore a really bad choice for hunting any kind of game animal. Hell on varmints though.
