The purposeful use of a ring as a weapon will open anyone for criminal charges under concealed weapons.
Using a ring to strike effectively, not just punching, takes specific training and continual reinforcement, as you would be striking specific points on an attacker for greater effect. Someone buying a ring and thinking this gives them the ability to defend themself instantly is making a very poor choice. They're more likely to get a better return bying a kobutan or carrying a strong pen and learning how to use it, than thinking a ring is the answer.
While there are a very few groups that have the potential of using ring striking, this is not a beginner option.
In fact use of any hand held weapon in most cases should be secondary to strong empty hand training (though the escrima arts are an exception to that answer).
Of course the most powerful hand held weapon any of us are likely to touch is a car, and I would never advocate using that either.
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