I've seen them mentioned on other Forums but never in person. It just seems to me it'd run afoul of the BATFE's "other weapons" category. I don't recall the exact language but something to the effect of weapons disguised as other objects being strictly verbotten (i.e. zippo and fountain pen guns). Since it's built into a knife it may be perfectly legal but I wouldn't be willing to risk Federal charges by owning one. Seems an NAA mini would be a better option and likely better quality as well. Just my humble opinion, hopefully others here can offer insight into the subtleties of the BATFE regs.