What typically causes a 'failure to fire'?

A 'failure to fire' typically occurs when there is an issue with either the ammunition or the operation of the firearm itself. This can include factors such as a faulty primer in the cartridge, whether due to manufacturing defects or age-related deterioration, or it may arise from improper chambering of the round. Additionally, it can also involve operator errors during the loading or firing process which lead to misfires or hangfires.

Operator error can encompass not fully seating the magazine, improperly loading the ammunition, or not following through with proper firing techniques. Ammo failure is also a significant contributor as it encompasses any issue that prevents the cartridge from igniting properly when the trigger is pulled. Thus, the combination of these factors makes 'operator error or ammo failure' the most accurate description of what leads to a 'failure to fire'.