I only ask because of an e-mail response I received from someone. He wrote as follows:
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It sounds like the pin that it projects into the barrel is retained by the cartridge case. That sounds a lot like the captive piston rounds that contain their own propellent gases. Such rounds are deemed to be suppressors by BATF (Small Arms Review, Vol. 5, No. 9, p. 48). Wouldn't that be a surprise if each purchaser were required to pay a $200 tax, get fingerprinted, get CLEO signoff, and wait several months to take delivery?
If the "safety bullets" are not found to be subject to NFA, what do you want to bet someone will glue a relatively light bullet to the front of the case, setting it up for the pin to strike when fired? Sounds like a field-expedient self-suppressing captive piston round, no?
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I've seen a picture of the captive-piston pistol rounds that he refers to. Very rarely seen, because the things are mainly used in covert military operations. Highly specialized stuff. H&K's underwater pistol (P11) might use a similar cartridge design, I'm not sure.
I found the second part of the above e-mail very interesting. If someone could modify your safety-bullet to prevent the case from expanding, they could make a dandy pistol round that's quieter than a .22 CB Cap. Hmmm...