One of the curious anomalies of cartridge design is the .38 Super. It came along in the late 1920s, when law enforcement was concerned about punching through the heavy steel of automobile bodies of that day. That is an understandable concern, but the means of getting it done is decidedly strange. The .38 Super is the same cartridge (dimensionally) as the earlier .38 ACP. It is just loaded much hotter.
It worked OK, but was never well known for accuracy. That's because it used an unusual semi-rimmed case that headspaced on a small shelf in the chamber. The Super got a major lease on life when Irv Stone of Bar-Sto developed a proper barrel that headspaced on the case mouth. I recall the day I went to the range with a new Commander in .38 Super. With the original factory barrel, the gun shot about a 12-inch group at 25 yards; with the Bar-Sto in place, it was about an inch. Colt's own barrels are now made to headspace like the Bar-Sto. By the way, never shoot .38 Super ammo in one of the original .38 ACP pistols.