It is due to both the H 110 and the short barrel. I believe it is a characteristic of ball powder. I used to get the same from shooting H 110 and WW 296 in a .357 Ruger Blackhawk 4 5/8 bbl. It was especially fun just at dusk. Boom! You could cook marshmallows over it! I have also got a fireball from using ball powder in a 7x57 rife. I think it was Ball C2.
As I recall, once another guy at the range actually came over and asked if I was OK after witnessing one of these fireballs.
Both H 110 and WW296 produced quite accurate loads with 125 gr Remington bullets.