The screenplay is an old project of Sylvester Stallone. It was written many years before but it was never realized and then abandoned due to his age. It came back into fashion when Jason Statham showed interest.
When Gator is talking on his cellphone in the lab, one shot shows him with his gas mask around his neck, and the next shot shows him with it on his forehead.
The shotgun Broker grabs appears to be a Benelli M4 12 gauge, a semi-auto shotgun (you can tell this by the way Broker loads it by pulling the charging handle on the side of the receiver). Yet every time Broker fires the shotgun you can hear the sound of someone racking the slide of a pump action shotgun.