Based on some of the videos/testimonials I've seen, it looks like with the right ammunition, they can cause pretty nasty injuries if aimed at the face with the heavy solid ammo. I think they would provide a reasonable deterrent after the first shot if the initial shot connects in the right place. If I were going to use one (which I wouldn't, just talking hypothetically), I'd likely only fire with the solid ammo in a situation where deadly force was justified.
They do look "gun-like" enough that someone who doesn't know what they are might assume they are a firearm which could provide some deterrent value until they are fired the first time.
They can shoot pepperball ammo, but I've seen pepperball ammo used in videos (using other launcher types) and I've not been that impressed. I'm sure it can be effective, but getting good results seems a lot trickier than pepper spray, which is saying something. The pepperball ammo (practical issues aside) could be useful in situations where deadly force is not justified but where pepper spray would be allowed.
If a person could not have a firearm for some reason, I could see how someone might choose one of these. I'm not saying it would be the best option for non-firearm self-defense in a situation like that, but it's also not a total joke from what I can see.
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