Very interesting thread, but I have found some contradicting data from Hodgdon's online data manual. Most folks here seem to agree that a slight bump in powder weight using 357 brass is beneficial, however Hodgdon appears to claim the opposite! I used their calculator and selected 148gr HBWC & HP-38 for both calibers, their results:
38 SPECIAL
3.5-4.0 grains, 869-956 fps
357 MAGNUM
3.0-3.4 grains, 845-908 fps

Note the MAX load for 357 is BELOW the 38 STARTING load ??????????
Very confusing to say the least! I think I've lost my faith in Hodgdon's calculator. Any thoughts?

Simple: The limit is caused by the HBWC; it has nothing to do with the powder pressure. So it is NOT a usual MAX load due to pressure concerns. In my experience , even a MINIMUM load of Green Dot can blow the skirts off HBWC!!
Added: And the more air space you have under a HBWC, the more likely it will blow the bullet into two pieces, maybe even sticking the skirt. It has to do with forming a pressure wave, like leaving an air space above a black powder ball, which can bulge the barrel. Internal ballistics do not lend themselves to intuition, or "common sense" based on what we think we know, but most don't. Using a short cartridge and seating the HBWC right down on the powder is safer.

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Science plus Art

I'm not speaking about the low limit and understand the fragility of the HBWC. In both examples I cited the bullet was a HBWC. My point was that Hodgdon suggests a heavier charge of the SAME powder with the SAME bullet in 38SPL cases vs 357MAG cases! Go figure???????

Read somewhere it's like 10% but I generally just play around midrange til I find the accuracy I'm happy with. You are shooting SP level loads in a Magnum so high pressure isn't an issue. 4gr of Green Dot under a 158gr LSWC has been working for me.

I use 3.1 grains of Bullseye under a 148 grain WC for my target 38 Special loads. If I was loading for 357 I'd do a tad less than a 10% increase in powder weight with Bullseye = 3.4 grains. Might start at 3.3 grains of Bullseye.

Besides what OKFC05 posted above, I can give you a most probable mechanical issue limiting the 357 Mag data with that bullet. What I think you are seeing with this data is due to the test setup they are using for both calibers. They are most probably using a closed chamber test barrel for testing both 38 Special and 357 Mag and most probably using the same test barrel to measure both. Since 38 Special cases are only physically different from 357 Magnum in length only, a 38 Special round shot out of a barrel that can chamber both cartridges would treat the 38 Special rounds as a closed chamber gun that has a very long leade or freebore and helps lower chamber pressures and possibly give a little more velocity, since the skirt will swell out to help seal the smooth bore section before the rifling.

I've been looking at different powders to try and am considering a ball powder like Hodgdon/Winchester 231 to use next. It's a little faster than Unique and readily available in my area. I've read that ball powders meters well so it's worth a try. This is from the Hodgdon site;

38 Special - 158 gr. cast LSWC - 7.7" barrel
3.1 - 3.7 grains 782 - 834 fps 11,900 - 14,600 CUP

357 Magnum -158 gr. cast LSWC - 10" barrel
3.4 - 5.0 grains 796 - 1,109 fps 12,600 - 23,900 CUP

In this case I think this pretty much bears out what those above have been saying. Bumping up the 38 starting load a couple of tenths seems to produce similar results. Using a 10" barrel on the 357 must skew things a bit but can't think it's huge.

Just clean your gun after shooting it, problem solved. I love the smell of Hoppes or Ballistol. My Old Model BlackHawk .357 loves cast 158gr .38 loads with Unique. Have a hard time switching to 357 brass when these shoot groups you can cover with a quarter ; at 25 yards.

You can use exactly the same loads in .357 cases as you use in your .38 loads. That smidge of extra length isn't enough in old revolver cases (which began with Black Powder loads) to make any difference in volume a problem at all. It's only purpose is to prevent loading a .357 into a .38 special gun.

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"He was kinda funny lookin'"

A less than one inch group at 25 yards? Young man, you have obviously never seen me shoot. 3 out of 7 just in the black is a good day

Forgot to mention that is with a well rested gun. not freehand. My shooting is more like yours freehand. But if the guns shoots good rested you cant blame It.

Nothing wrong with just loading the long .357 cases for the weapon.

In my 686 6" a " .38 special load " in my weapon tested out at;

110 Jhp Red Dot 945fps to 1305fps with Unique powder.
135 Jhp Unique 860fps to 1035
158 Lwc Trail Boss 755fps to 1014 with Unique

Sorry, I do not shoot the 148gr out of my 357 Magnum revolvers, which are reserved for my J frames.

I do not hot rod the little 110 Jhp in my .357's and seldom use them even from the 1254
to over 1500fps that I did test out.

The 125 Jhp is the minimum weight that I think, works in this weapon from a full load of w231/CFE
fast powders to SR4756 and IMR2400 for loads from 1450 to 1500fps.

158 Lwc loads starting with Trail Boss around 790fps for target work
up to 1100 to 1200fps Unique powder loads for longer range work to
full loads around 1320 for hunting but these may start leading the barrel.

The 158 XTP bullet with IMR2400 can get up to 1340fps as a second choice for a hunting load.

For general plinking loads with a Jacket coating you might try down loading the Hornady 140 gr bullet.
They shoot great in my revolvers and the price is right.

Ps;
with a .357 case you are actually shooting a "Reduced" 357 load, not a .38 special load.
Carry on.