I am not an expert on the Mac 10 but the fact the bolt was locked back indicates it was a full auto gun which fires from an open bolt.
I probably would have done the same thing you did but it was probably not the best option. The person who suggested you get back in the vehicle and leave, then phone was imo giving the best advice.
I'm not an expert on Macs either but I know an open bolt Mac style gun with brass glistening when I see it in a guy's lap 3 feet away.
Whether the semis shoot open bolt first round or not I haven't a clue but I'll bet you my life and yours that somebody on this thread knows for sure..
LOL....that's not a bet anyone would take is it?