http://www.us.army.mil/suite/photo/ako-400-20160518-262446054549276c3292fb.jpg



http://www.us.army.mil

You mean the sparks flying?  That's what happens when you shoot guns at night.  Itsn't usually captured on film, but it's pretty common.  Something that really impressed me with the movie "13 hours" is that they included the sparking on the Technicals.  (older MGs and russian guns do it a lot worse than the modern AR family)

Is that metal particles or burning powder?

My guess is metal.  Excessive powder leads to more flash which you can already see in these images.  Just the bright burst by the muzzle.  The sparks seem more like fine metal particulates that are combusting, but I'm not certain.  The fact that russian guns tend to do this more than western weapons would support this idea since AKs, PKMs, etc tend to be loaded with steel cased ammo with harder bimetal jackets.  Harder jackets mean more material is removed from the bore and get thrown downrange.  Heavy friction makes it hot which turn the metal bits/shavings into glowing sparks.  Not certain this is 100% right, but I'd bet money that I'm at least close.

burning powder, you see that a lot more when shooting lower spec ammo.  Wolf for example does that very well both in 7.62 or 5.56.

Uh, that's a DOD pic, so would be M855 or M855A1.