They are exactly the same case, only difference is the load. The case head is .405" while the body is .385", so it is a semi-rimmed cartridge.

The Colt's Government Model 38 Super Auto barrel, like the 45 ACP barrel, does not have any support for the case head/rim. It headspaces on the case mouth, like the 9mm Parabellum.

Pics attached. In the two top views, the cartridge is pushed into the chamber against the case mouth. In the bottom view, the extractor is holding the cartridge against the breech face. You can compare the views to see how much difference there is.

And the .45ACP also headspaces on the case mouth, not the rim.

Personally... I think the barrel you have illustrated has excessive headspace... even if it came that way from the factory... it looks like the chamber was cut too deep. You might consider getting a new .38 super barrel, and rechambering that one for the 9x21mm cartridge that is prevelant in Europe in those countries where guns chambered for the 9x19mm (Luger) cartridge is prohibited because it is a military caliber. Italy is one of those countries...

Heck, the 9x21 may even be a better fit than the .38 super in your present chamber!

I have a new 6" luger barrel stamped "38 Super" on the outside, but I agree that the 38 Super semi rimmed cartridge would not be the best feeding one from a luger mag. It would most like work better with 9mm Largo (9x21) or 38 Auto without any modification to this barrel. TH

I happened to be thumbing through Fred Datig's The Luger Pistol [1955; reprinted 1958] and on page 196 noticed the following paragraphs concerning the ammunition supplied for the 1907 tests, posted here for your consideration...