PMC rifle ammunition is pretty flashy to begin with. This picture shows shot three or four in a sequence of rapid fire. By then, the build-up of hot gas and unburned powder particles caused a brighter muzzle flame than before.
The rifle is PWS Mk.2 piston .308 with a Gemtech suppressor. The shooter is Yih-Chau Cheng, Oakland Gun Rights Examiner.
Worst I’ve ever used was Remington 9mm hollowpoints; biggest flash of any pistol ammo I’d ever fired.
VERY cool photo.