Lemming +1
The type of powder used also can contribute to sooty deposits. Some ammo brands will shoot cleaner than others at low pressures.
As said, not a safety concern if the bullet is hitting the target.
E.G>: My light competition light loads leave some sooty residue on the cases and in the pistol, especially with Winchester 231/HP-38 powder. With full power loads there's very little residue.
Yep.
I'm not that familiar with .380 ACP factory loadings. .380 has a maximum average peak pressure of 21,500 psi, as opposed to 35,000 psi for 9mm Luger. I suspect that many manufacturers use the same or similar powders for .380 as they do with 9mm. This would translate into more residue from the powder charge combusting incompletely.
Most handloaders use the same or similar powders for .380 they would with .38 Special +P (18,500 psi). If you compare the case volumes of .38 Special, .380 ACP, and 9mm Luger, you will notice that .38 Special cases are significantly larger (due to the chambering's black powder origins). Commercial manufacturers are not limited to the powders marketed to handloaders. While they could contract for a powder specifically designed for .380, it wouldn't be economical. Since most of the powders used in .38 Special commercial loadings would be too "bulky," I suspect that they are using powders which are too slow-burning for 21,500 psi (i.e. more suited for 9mm, .40 S&W, etc.).
Some of this is speculation on my part, but it is consistent with my limited experience with .380 ACP and U.S-manufactured 9mm Makarov loadings. (Eastern European 9mm Mak loads seem to use powders developed for that chambering/application.)
