Did your can manufacturer tell you that or did Kel-Tec?I was told not to use mine unless the barrel is at least 18 inches. That might only apply to my can. I really don't know.
I didn't see too much back blast hitting his face. But maybe.Make the video larger and focus on the shooters face. He's getting a face full with each shot. He even seems like he flinches from the back-blast.