............This was from "Handloader Magazine" #116. July-August 1985. Note that this was NOT an article about shooting cast in the 38 Super. Of the 34 loads shown (30 reloads, and 4 factory) all but 2 were jacketed loads. The 2 cast loads were also fired in conventional 38 Super brass.
The basic reason for using the 223 case for forming Super, 38 Super brass (they were calling it the "Super 9" was because of the common headspacing issue the Super was afflicted with. This was a means to make a case long enough to truly headspace on the casemouth. In addition, due to the difference in the rim between the two cases, it may be necessary to modify the extractor. The author's gunsmith altered his and it worked with both conventional and mod'ed 223 cases.
Also a matter is chamber ID and the barrels groove if shooting cast. It is possible that the 223 cases may have to be inside reamed. Finally the author found that using small rifle primers worked better, due to their height difference in a case designed for small rifle primers.
The Basics, using commercial 223 brass
1) Cut 223 Rem cases to 1.1"
2) Trim to approximately .905" using a .33 pilot*
3) Expand in 38 Super die
3a) Ream with .356"-.358" reamer if using .357" bullets or boolits**
4) Run through FL size die
6) Chamfer inside and de-burr outside of case mouth
7) Prime with small rifle primers
* Experiment with final trim length as after firing, the expanded case will shorten slightly
** This will depend on the brass, chamber ID and your projectile diameter
Some load data. Barrel length 4-1/4". Remember, this was 23 years ago
90gr Horn JHP, WW540 11.5grs, 1770 fps. Working load.
90gr Sierra JHC, WW571 13.3grs, 1816 fps. Max
115gr Horn JHP, Unique 7.7grs, 1460 fps. Low Velocity
115gr Horn JHP, WW571 12.1grs, 1650 fps. Max, full case.
125gr Speer JSP, Unique 7.9grs, 1390 fps. Full case.
125gr Speer JSP, WW571 10.5grs, 1450 fps. Max, full case
If you're inclined to heavy slugs in the Super, Layne Simpson had an article in "Handloader Magazine" #158, July-Aug 1992 Titled (for some reason) Heavy Bullets in the .38 Super.
34 Loads (jacketed) with 147gr Hornady, 150gr Noslers, 158gr Speer, 160gr Hornady, 170gr Sierra, and 180gr Noslers. Velocities from 1228 fps to 1012 fps.
23 Loads (cast) 147gr TCFN Bull-X, 150gr SWC Bull-X, 155gr SWC Bull-X, 160gr CS RN. Velocities from 1238 fps to 1150 fps.
Another Layne Simpson article in a 1996 "Handloader" on the 38 Super has load data for 4 load levels of intensity, and DOES show that the modern 38 Super IS pretty SUPER