Which of the following is not a safety feature of firearms?

The recoil spring guide is not considered a safety feature of firearms. Instead, its primary function is to support the recoil spring and ensure proper cycling of the firearm mechanism during operation. While it plays a crucial role in the functionality of firearms, it does not contribute to preventing an accidental discharge or enhancing the overall safety of the firearm itself.

In contrast, decocking levers, firing pin blocks, and half cock safety notches are all designed specifically to enhance safety. For example, a decocking lever allows a shooter to safely lower a cocked hammer without firing a round, while a firing pin block can prevent the firing pin from moving forward unless the trigger is pulled, thus reducing the risk of accidental discharge. The half cock safety notch provides an additional safety mechanism that prevents the hammer from falling all the way, further reducing the potential for unintended firing.