One part criticism, one part methodology, Violence of Mind pokes at...
Survival is all about doing what you must to stay alive, and using the resources you have to their fullest. Sometimes, that means preparing to defend yourself with purpose-built weapon systems, and other times it means making the best of a bad situation, and using an improvised weapon for self defense.
Sure, improvised weapons may not be as effective for self-defense as firearms or blades, but they have one major advantage: they're covert. These makeshift tools are also permissible in many locations that restrict traditional weapons, providing you with the means to defend yourself in a variety of settings.
A disclaimer: obviously, before you consider carrying any item for self-defense, check your local laws very thoroughly. Even if it's fully legal to carry in your area, only use it in a worst-case scenario to defend yourself, and be prepared to explain your actions to law enforcement or a judge if necessary.
Carabiner knuckles are a convenient improvised self-defense tool. Most of us know of the potential that brass knuckles, knuckle dusters, or “knucks” have to inflict massive damage with a single strike. These purpose-built knucks (and even so-called “decorative keychains” or “paperweights”) are typically restricted by local weapon laws, similar to blades and firearms. However, carrying an ordinary carabiner on your backpack, water bottle, or keys is generally legal.
If you were to use a carabiner for self-defense, you'd want one large enough to fit all four fingers comfortably. You'd also want a locking collar, to prevent the carabiner from snapping open and catching on your skin. Finally, high-strength aluminum climbing carabiners will likely resist most impacts, but if you're looking for extra knuckle protection, find a solid steel carabiner.
We'll say it again: be responsible. Don't do anything dumb or illegal with this knowledge, and don't blame us if you do.
That said, a sturdy locking carabiner is a great addition to any EDC or bug-out bag, whether you use it to rappel, retain gear, or protect yourself.