I am new to the forum and to my M&P 15R and need some help. I bought a 1080 Round Can of Surplus Russian 5.45x39 ammunition. Features a Steel core 53grn full copper plated steel jacketed bullet, lacquer coated steel case, and berdan primer. It seems that every 4th or 5th shot fails to fire. I can see the primer has been struck. Any ideas on how to fix this? I greatly appreciate your input.

Thanks!

Do you try to fire them again and they work?
I mean if there is a dent in the primer then it seems it should have fired if it was good.
Bad ammo or a light strike by the firing pin.

I did not try to fire them again. The puzzling thing is not every round fails to fire, just every 4th or 5th.

You might try taking the bolt apart and cleaning out the firing pin channel w/ pipe cleaners. Maybe its gunked up .