The component's of 777 are charcoal , potassium nitrate , potassium perchlorate and graphite , the change from bp is that the sulfur is replaced by the perchlorate and that is to speed the burn process up now the sulfur has gone but by replacing the sulfur the mix is not classed as an explosive but a propellant so no licence is now required.
The perchlorate is very good at attracting water ( hydroscopic ) so keep the containers closed as much as possible.