What mechanism describes a firearm where all rounds are fired in double action, with no capability for single action?

The mechanism that describes a firearm where all rounds are fired in double action, with no capability for single action, is double action only. In this type of firearm, the trigger pull both cocks the hammer and releases it, resulting in the firing of the round. This means that whether the chamber is empty or loaded, the trigger has to be pulled fully for each shot, providing a consistent and longer trigger pull each time.

Double action only firearms are designed for simplicity and safety, as they don't have the option for single action firing, which can allow for a shorter and lighter trigger pull after the initial round is fired. This is particularly advantageous in self-defense situations where speed and reliability are critical. The absence of a single action capability also helps prevent accidental discharges, as the trigger pull is inherently heavier.

In contrast, mechanisms like single action only or double action semiautomatic introduce different operational nuances that allow for single action firing under certain conditions, which are not present in a true double action only firearm. Striker-fired designs can have similar functionality to double action mechanisms but don't strictly fit into the same category since they have different internal firing systems.