I'm not saying it wouldn't work, but I remember several years ago reading a news report about someone altering a flare gun to shoot 12 Gauge rounds. The report ended with the man losing the use of his right hand when the flare gun blew up.
I'd be wary about firing one.
Disclaimer: When I was younger I did many things people would possibly qualify as stupid and life threatening, such as building a .22 zip gun out of a car antennae. But I survived doing that undamaged, so I consider it more of a valuable learning experience.
So I'm feeling slightly hypocritical posting this.