A few years ago I was shooting at some 3mm sheetmetal from about 25 feet or so, I had it angled slightly to the side to deflect the rounds a bit and the 40s&w was still going right through it.
I switched to 45acp and on the third shot the copper jacket of the FMJ round came back and chewed on my right forearm a little bit, just a scrape.
I can only figure it hit a dented curve and somehow the jacket made a U-turn.
