The "go to" powder for magnum level loads in .357 for generations has been 2400. And it does great work in carbine and rifle length tubes too.
As others pointed out, the small velocity difference you are seeing in those particular loads are due to the fast powder. 231 is a great powder for small capacity cases and barrels less than 10". Its gain from longer barrels is proportionally less than slower burning powders.
296/h110 and AA#9 are good for top end loads as well. Note that these powders don't perform as well in light charges and short barrels. Even 4" is too sort for real efficiency with the slow powders. But, get to 6" and you've got something, and above 6" you get more and more.
Be aware that the heavy loads where the slow powders do their best work are NOT suitable for all guns chambered in .357 Magnum. And sometimes its not just the small light guns!
some time back, a friend brought over a couple .357s and a box of ammo, 125gr JHP and a case full of 2400. It was a load listed in his Speer manual, and not the max, either, but up there...
I added a couple of my .357s and the chronograph. The guns were S&W M19, 6", S&W M28, 6", Desert Eagle 6" and Marlin 1894 carbine, 18.5"
the first gun shot was the M19, and the shooter doubled the gun. TO this day, I don't know just HOW he did it, but he fired 2 shots not the 1 intended. The chrony said 1620fps!
At that point we decided to discontinue shooting the M19 with that ammo. The four unfired rounds fell right out, but the two fired cases could not be extracted by hand, and had to be driven out of the cylinder with a rod and hammer!!
SAME ammo, M28, six shots fired normally, cases ejected by hand normally after a slight resistance. Chrony say 1670fps.
Same ammo out of the (nominally) 6" barrel of the Desert Eagle. (And that DE also had the polygonal rifling). Full magazine fired, function normal and flawless. Chrony reading 1720fps!!
Same ammo fired through the Marlin carbine, clocked 2200fps.
With the right powder you get a significant gain from the rifle/carbine barrel length, possibly as much as 50%. With the "wrong" powder you do get an increase but its only a fraction of the increase you get with the right powder.
Fast burning powders are not the right ones for max velocity in longer barrels, they simply "top out" too soon.
If you're a car guy, think of it in terms of rear end gear ratios. A 4:11 gear ratio is really fast off the line but has a lower max top speed than a 3:86 or something like that. Back in the 70s, a friend had a tricked out El Camino, with a 327 Corvette engine and a 4:11 rear end. That was one quick car. Nothing could touch it in light to light street racing and it was very fast in the Quarter Mile. But on the highway or long track its top speed was in the 90s, and my 1960 Desoto (totally stock, and with a 5000lb registered curb weight) would eventually catch him, and leave him behind...
Like my old Desoto, the slow burning powders take more time (barrel length) to get there, but when they get there they are significantly faster than the "quick" powders which reach their peak pressure sooner.
