I think this is possible, however the probability is very low.
I did the search on this case, and one article mentioned that he used the "assault rifle" for target shooting. I couldn't find more details, but some of the surplus ammo made with the bimetal jacketed bullets. Some of the surplus ammo is steel core. Russian 7.62X39 is a good example; steel jacket is copper washed and called bimetal by the manufacturer.
He did call almost immediately to report the fire. So, here we have very unfortunate chain of events.
