During my time in Vietnam, the only failure to fire I experience was after I'd disassembled to lower receiver for cleaning (this was either strongly discouraged or prohibited). When I reassembled it, this genius neglected to put the legs of a spring over the retaining notches on a pin. The pin drifted out the next time I fired a magazine and the weapon locked up. Entirely my fault.
Stateside, I never had a failure to fire or to chamber a round.
Thank you for your service to our country, my FRiend!