I've got a 3" model 64 NY1. I really wish it was a 357, but the buttery DAO and general handiness keep me from parting with it. Guess I'll feed it Underwood Ammo and keep it for hard times.
Well,...far be it from me to suggest non standard loads. But if you're not hesitant to shoot factory "warm" loads (Underwood) in your modern K-frame 38s, you might find 7 grains of AA#5 over 158s to be very satisfying. It's just a little tweak above what the Speer manual calls a +P. (They call 6.6 grains of #5 with swaged lead 158s a +P load that comes in under 20,000 PSI.)
Graf & Sons have the Hornady swaged 158 grain SWCHPs in stock for a reasonable tariff. My foundings with AA#5 tells me that the Hornady 158 grain swaged SWCHP over 7 grains of AA#5 in your modern K-frame would result in a right respectable FBI load.
My opinion, a Modern K-frame .357 (model 13 & 19) aren't even a bit stronger than a modern K-frame .38,...and people have been pushing 35,000,..even 40,000 PSI through K-frame .357s for a long time. (full tilt .357s will loosen up a model 13/19 toot sweet, but they won't blow one up)
The 7 grain AA#5 load with 158 swaged bullets is probably way down around 21,000 PSI. Way below the loads that people have been shooting through Model 19/13s for many years.
As for all that mumbo jumbo about K-frame .357s being made out of superior steel than the Modern K-frame 38s,...bullshit. All the magic about tool steel has been over since the early 60s. All typical tool steels cost about the same. From a manufacturing standpoint, it wouldn't be cost effective to fabricate essentially the same revolver out of two kinds of steel. Just pick the best one and go with it,....same machining characteristics,..same heat treat procedure. Why would a manufacturer complicate the manufacturing process by using different steels when the cost of all tool steels are essentially the same? It makes no sense.
Back in the old days when new steels were first coming available, firearm manufacturers experimented with different steels for their revolvers. But the old days were a long time ago.
Other than a few cosmetics and the depth their cylinders are bored, there's no difference between a modern Model 10s and modern Model 19/13s.
S&W soon learned that a K-frame .357 wouldn't hold up very long with 35,000 to 40,000 PSI loads. That's what caused them to produce the 586/686 series.
But both a modern K-frame .38 *and* a modern .357 K-frame will handle 21,000 PSI all day long.
21,000 to 25,000 PSI is a good place for .38/.357 loads to be when they're shot through a modern K-frame S&W. 35,000 to 40,000 is too much for them regardless of what the stamping on the barrel says,..and the old standard 17,000 PSI loads aren't enough to realize their potential.
7 grains of AA#5, 158 grain swaged Hornady SWCHPs. You'll like them.