I'm finding about 4" at 50 yards is becoming the norm, so now it's time to reach out to 100. The time I did, it became clear that I was going to need to increase my velocity and should investigate something a bit more aerodynamic than a 148 wadcutter or 158 RNFP.
4.0 of Bullseye behind a 158 RNFP was what I tried and both dispersion and drop were fairly awful. A 125TC in front of 5.1 of B/E was a bit better, but still not very satisfactory.
So, pet loads? I'm interested in things suitable for earlier guns (1905), as well as model-10,14, etc. I'm not interested in unduly stressing anything so I'm leery of Elmer Keith recipes and such.
Obviously, this is a job better suited to a .357 magnum and I'm sure I'll get there at some point.

Just a suggestion, but if you're using open sights, consider doing your benchrest testing at 25 yards with ten-shot groups and then test the promising loads farther out. I'm pretty sure the heavy bullets will do much better than the light ones when you get past fifty yards.

Lots of 158 SWC designs; try several if you can. One that might do a little better at distance is the Lyman #358429 design at 170 grains (ww alloy). I've never found heavier than that in the .38 or .357 to be very productive accuracy-wise. One exception might be the LBT 180 FP, but these might be hard to come by unless you cast your own and have the mould.

I'd also consider trying #2400 powder in either cartridge. Magnum primers in either may not be recommended by some, but I've seen some accuracy benefits with magnum primers in the .357. Compare standard and magnum side-by-side if they're available to you. Good luck-

My standard load for long range in my S&W 38/44 Outdoorsman's is 160 gr cast 358156 HP SWC with 11.5gr of 2400 with a magnum primer. It comes out of my 6 1/2 inch barrels at just around 1050fps.
Might be too warm for the 1905 your mention. As a matter of fact, I would not try to stretch that 1905 era gun to 100 yards with anything other than standard 38 Special loads.

I like the rnfp entering the forcing cone without shearing any lead. I believe if your groups work at 50 the same loads will work at 100, That means your sighting system should be looked at rather than your loads. EK had a gold line sight. IOW, he would put a gold line on his front sight, put the top of the sight at a 6 O'clock hold, and the top of the rear sight at the gold line. If you have another method to make your holdover the same each time use it. That might be a paper plate on a stick stapled over your target with an X on it. You might need a paint stirring stick or a yard stick to attach to the target backer. Most load manuals will show drop at 100 yards.

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Front sight and squeeze

I shoot all the time at ranges out past 200 yards with my S&W revolvers. In the 38 Special I like any of the 158 grain cast bullet designs at 800 fps, although aerodynamically, a round nose design is the best choice. My Model 14 is my 38 of choice for longer range shooting and it's 6" barrel gives an adequate sight radius for long range and optimal velocity for 38 Specials.

In the .357 Magnum, I use a 180 grain RNFP cast bullet design at 950 fps for excellent performance at the longer ranges. When fired in 6" N Frame Model 27s or 28s, it is a fun and accurate load. That said, I also use a 147 grain 9mm cast bullet design sized to .358" at 1,100 fps and with it's pointed meplat, it performs as well at the heavier bullet. These are probably considered mid range loads in the magnum.

Good bullet stability for accuracy at longer ranges is a combination of its ballistic coefficient, its bullet length, its RPM (dependent on the rifling twist and velocity) and its decayed or impact velocity at the given range. This is probably why you find accuracy issues with the WC designs past 50 yards. When shooting on the PD pistol team, I shot literally thousands of 148 gr HBWC at 50 yards. In stock S&Ws with their slow twist (1 in 18.75) it was quite common to see slightly oval bullet holes in the B27 target at 50 yards indicating the onset of instability. Colt revolvers (1 in 14 twist) had perfectly round holes with the same load. It is all about gyroscopic stability and the Colt's faster twist kept the slow and long 148gr HBWC stable for the 50 yard stage of fire.

I shoot my revolvers a lot at the longer ranges, so I don't stress them with hot loads. Instead, I concentrate on technique for my position, sight alignment/picture and trigger control. By raising your front sight blade in relation to your rear sight you can compensate for trajectory at longer ranges. With practice, it's a cinch to do. The longer the range, the higher the front sight above the rear sight. Sight radius, velocity and bullet design will all have a bit to do with how much.

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Keep your powder dry.

At our range, shooting at a metal Ram at 100 yards, with a 6" barrel.........

Shooting at the target, the 38 special would hit the dirt 80% of the way to the target.
The .357 magnum would hit the target with just a little drop.

With a 38 special, you need to have high fps and also know how much drop
the 110 to 158 grain bullets have, to get enough elevation, to hit the target.

Have fun.

I do try loads at 25 then 50 before trying anything at 100, but there are many loads that will shoot very well at 25 that fall apart badly when stretched to 50, so it's not the quickest process.

Indeed, primer selection is a bit mixed just now, but I do try a couple types.
I'd have thought round-nose would be more aerodynamic than a SWC, but happy to give it a go.
As to weight, I'd have thought that lighter (and thus faster) projectiles would spend less time in flight and so would dodge wind better, but content to be mistaken. Of course, it's critical they don't start supersonic and go trans-sonic during flight, so there's that to worry about.

I've got some larger targets, so I don't mind drop and don't need to fuss about holdover for this (for now), just need some better groups.
I'm not trying for anything very fancy, duplication of the ~8" groups I see from a good 9mm with irons would be perfectly acceptable.

With standard 38 ammo with a 25 yard zero........ drop at 100 yards est.

110 xtp.... 945fps........................ -14.7"
125 xtp.... 900 ............................. 16.1
158 Lawman... 850 ....................... 17.6
158 LRN ... 760 ............................ 22.7

This will depend on the hight of your sights on a 6" barrel.

I will place the base of the front sight in the rear sight notch for added elevation
to help "Drop" the rounds onto the target.

Have fun.

At 100 yards perhaps the OP's talking about a rifle...?

Cheers!

I have found that the standard rifling twist of Smith revolvers doesn't work very well past 25 yards for light target loads. The 2 ways to spin a bullet faster are: more velocity in the same barrel or same velocity, different barrel with tighter twist.

That's why all the PPC guns had custom barrels with 1 in 10 or 1 in 14 twist. Then the bunny fart wadcutters would stay stabilized out to 50 yards.

Sub sonic is approx. a little under 1100 fps, depending on a number of variables. If your target loads are 900 to 950 fps, for 50 yards or 1000 to 1050 for 100 yards, you will get the best groups you're going to with a stock barrel in .38 Spl. You may well get better groups going faster with a .357 load.

Also, heavier bullets retain their forward and rotational velocity longer than light bullets, so will shoot better groups at longer distance. I have always had better groups with hollow points than solid points.

These are general guidelines for a starting point. There are many variables, and some bullets will shoot better groups than others, everything else being equal.

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protocalldesign.com

I've been shooting my revolvers from 50 to 200 yards in Met. Sil. for 40 years. We don't need no stinkin' rifle!

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protocalldesign.com

I think the 158 grain round nose would work best. This isn’t a 2,000 FPS rifle, so hollow point won’t give you an advantage. I have found the round nose bullets to be most accurate overall.

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and what his trumpet saith

I will agree on the need for increasing the speed of the bullets and dropping
the 148 wc bullet for the heavier 158 lead bullet or some 110 or 125 jacketed bullets.

If you are shooting a K frame or heavier frame 38 special, the standard
158 gr bullet will work if you add 200fps to it with a near full load, with any
powders that you have on hand that will not go over safe pressures.

JHP or plated will also work if you have some on hand , to try out.....
a full +P load is not needed but at 100 yards it won't hurt, if they work
in your weapon.

Do you own a spotting scope ? ! (big grin)

13.5 grains of 2400 and Elmer’s bullet works for me!

Alliant has +P load for 38 spl using 2400. That would be my first approach. Pay attention to the revolver however. Some won't handle that pressure very well, or maybe not at all.

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That's just somebody talkin.