Which of the listed features is used to ensure that a firearm does not discharge unexpectedly?

The feature that is used to ensure that a firearm does not discharge unexpectedly is the firing pin block. This safety mechanism is designed to prevent the firing pin from moving forward and striking the primer of a cartridge unless the trigger is deliberately pulled. By implementing this device, the firearm significantly reduces the risk of accidental discharges that can occur from bumps, drops, or other unintentional actions.

Trigger modifications, while they can affect the sensitivity and pull weight of the trigger, do not inherently prevent an accidental discharge. Similarly, an ergonomic grip enhances comfort and handling but does not contribute to the safety mechanism that prevents unexpected firing. A reinforced barrel is essential for handling higher pressures and improving durability but does not serve to guard against unintended discharges. Thus, the firing pin block is the most relevant feature to address the issue of unexpected discharges in firearms.