I could go into it, but it'd be (even more of) an essay.
Getting old military surplus stuff is generally pretty safe - IE old AK ammo from russian satellite states, if you buy it by the tin/case, sealed, because those were cold war stockpiles and were stored in such a manner that if something kicked off then, say, Romania could rock and roll at the drop of a hat with a full arsenal of small arms.
The problem comes when you start getting into vintage or rare ammo, where it can be hard to source and you need to be pretty careful. So, for example, getting ammo for a 1950s Czech automatic pistol in a weird calibre that was never approved for military use would be more risky than getting good old 50cal BMG that everyone's been using in their light machine guns for near a century.
The 50cal SLAP rounds he used are really pretty rare, and are interesting from an engineering standpoint, but demonstrating them is fairly pointless; the US Military dropped them because they weren't really any more effective than boggo 50BMG, and they had a nasty habit of blowing guns up. But with 20/20 hindsight, given that he only had a dozen or so of them as far as I'm aware, he really should have reloaded them by hand to whatever the military spec was before firing - or got someone who could hand load them do that for him.
The lack of provenance is part of the problem, the fact these are rounds that are notorious for causing barrel blockages is the other. The two together really should have caused alarm bells to ring, and I bet they will now, for him and for others who are interested in old, weird ammunition.
