I'll admit I did buy a couple Weekly World News back in the day, but I was never a subscriber. So that qualifies me for speculation.
I'm assuming it was a real bullet as it sounds like it went through someone. So where did that bullet come from? Most likely same deal as Brandon Lee? I would assume they were doing closeups so they may have had real slugs on the set. But it is not outside the realm of possibility that this is foul play- people can be nuts and do crazy shit. Someone loads the gun planning to use it later for suicide or to threaten someone and then it gets moved. The movie was about a homicide accident- maybe Baldwin did go nuts with affluenza and needed the high only killing someone can give and figured they would never suspect him!
The other big safety issue is one that reminds me of something that could happen in outdoor situations- someone who is not an expert acts like one.