Modern military and some commercial ammo is very low flash and modern muzzle devices can dissipate it further just for this purpose. Not seeing a muzzle flash from an automatic weapon does not indicate anything.
Scale question too. Individual room lights hard to make out at camera ranges I’ve seen so far. Been looking for muzzle flash myself, no joy.
General reaction I’ve seen from operators, regarding giant-cap drum reliability is ridicule. Whatever this was, it ran pretty steady. Next time I see or hear the slowdown then ? FTF, I’ll link it.