Nice video, What ammo are you using?
If you are just trying to get a big muzzle flash, find you some Czech white tiped rounds with the 44.8gr bullet. It has almost no kick, and gives a big muzzle flash because the lite bullet is out the end of the barrel before all the powder is burned. It will double the muzzle flash in your video.
I find it at local gun shows, I think the last time I bought a box it cost $2.50 for the 12rd box. You can tell what it is by the lite blue box with the white stripe. It is short range ammo, it only good to about 200yds.
Czechoslovakian 1964 7.62x54r Hollow Core Short Range
Only complaint I might have is it looks like the camera man was a little too close to the muzzle of the rifle. Other than that very good vid, way to capture the fireball