Capot, sorry mate, you are miles off track. Most older sub-machine guns operate on API, advanced Primer Ignition. You have to be very unlucky to drop it butt first enough to get the bolt to fall back enough to clear the back of the top catridge but not enough to reach the sear. If you did manage that feat, then yes, it might fire ONE off, but the recoil would actually be slightly more thna usual, due to the reduced momentum of the bolt accelerating for a shorter distance, and thus would easily clear the sear and stop in the loaded and cocked back state.
Many other SMGs will do precisely the same, from UZI, to Ingram M10/11, L9 sterling of even MP38/40 types. The MP5 Heckler&Kock series will not as they have a closed bolt delayed recoil action.
Revolvers generally do not have a heavy enough hammer to be able to fall back and operate the cylinder rotating mechanism as well. The worst I'd expect any revolver dropped hard butt first to do was half cock.
Self loading semi-auto pistols are also difficult to discharge by dropping, even M1911 colts. However it is easier to achieve with older guns. M1911 types use a firing pin that is shorter than it's hole, so the lowered hammer will push it flush with the chamber face. The momentum of the pin carries it through to fire the cartridge, and it will then retract to allow the rear of the cartridge to fall with the barrel in the short recoil swinging link action that the colt uses. The (Modern?) 1926 designed Browning Hi-Power uses a cam but similar pin.
Newer designs with interlocks that positively lock the pin from moving under impulse, inclue Walther types like the P38, and earlier models too. It's not just fancy half plastic ego-9mm weapons that are well designed, plenty of old museum quality weapons were thought out properly and designed to be carried safely too.
I've been out of it too long to really understand the workings of modern Glocks and Smith&wesson Semi autos, but don't knock the older guns (literally) unless you know them well.
I'd also wonder about a possible ammunition fault in this case. Too easy to blame the handler, especially if he's American. The jolt from loading, or the jolt from dropping a safety lever which would lock the pin in place then drop the hammer on a modern double action gun, shouldn't cause a cap to ignite, but it has been known.
If it was handled properly, I'd expect a flat slug and mess of sooty paper in the bottom of his flight case.......