Nice mystery, but factually off. Black powder makes a LOT of fouling, an automatic pistol isn't going to be test/practice fired with it and be expected to keep operating past a few shots. Depending on the pistol it might get as much as 12, but it would require a LOT of cleaning.

The old black powder revolvers, '51 Navy, '60 Army, etc, that were designed with Black Powder in mind could be counted on for 12 shots before the fouling made them inoperable. Those revolvers had a LOT less parts to be affected this way.

I honestly expected the deformation to be the key clue. I'm not an expert, but it seems to me that a round hitting the back of a practice range is going to deform differently that one hitting a human body, even if it's through another object like a car seat.