7x57 FN49, Sounded like a MG42. Five rounds, BRRP.
Short stroking due to incorrect gas system adjustment for that ammo.
Good thing the rifle is heavy for the caliber.
My Izh35m was slamfiring one extra round ever now and then. Worst was when it haapened on releasing the slide to loaded the chamber. That get's everyone attention during a competetion.