Locks

Locks solve the access problem, not the theft problem. A cable lock through an action or a trigger lock on a firearm prevents a child or a guest from operating it, and prevents essentially nothing against someone with tools and time. That is still worth having, and in several states it is required.

A cable through the action, with the action open, is the more reliable design — it physically prevents chambering a round and is visibly secure. Trigger locks clamp around the trigger guard and on cheap examples can be defeated or, worse, can contact the trigger while being fitted. Never fit any lock to a loaded firearm.