Re small claims court: you could probably do it yourself if you have the time and the inclination.
A couple things to consider:
1. was the transaction interstate? This raises 2 issues:
a. You might have trouble hauling the seller into court in your home state, i.e., a jurisdictional issue.
b. How gun-friendly is your jurisdiction? If it's friendly, then it might make more sense bringing him into court than if it's not friendly. In an unfriendly jurisdiction, you can be sure the judge will find a way to f'k you over and make you lose the suit, at a minimum, or try to have you incarcerated (for bringing a gun to the courthouse) at the outside. The judge might also be ignorant of the law on private sales of parts and, indeed, what constitutes a "firearm"; explaining that the law makes only receivers firearms (under federal law - your state's law may vary) is likely to be too much of a mental stretch for the person wearing the robe. Small claims judges' attention is most centered on moving cases, not on fine legal points.
2. You'll need an expert to give testimony that "these are not genuine", i.e., the subject of this thread. While a letter from a recognized expert might do, you cannot be sure the judge will not require live testimony from your expert. Shipping experts around gets expensive and their time, moreso.
3. If you're going to send a letter demanding the seller undo the transaction - which appears to be all you really want - be careful in how you phrase it. Threatening to sue, to haul him into court can, depending on the jurisdiction, be construed as extortion. That depends on how the state writes its laws on that subject. Probably best to say something along the lines of "I've contacted recognized experts in the field and they are uniform in advising these barrels are not what you said. I propose we undo this transaction and both walk away wiser." Send it certified with a copy by regular so he can't later say "I never got any such letter". (They always do that.)
4. Be prepared to have him call any bluff you might make.
5. If you decide this is too much like work, then it's best to write it off to experience. Further, to get someone to eradicate the offending text on the barrels so no one else suffers. Whether you want to tell who the seller was is your call. I'm glad I'm not in your shoes - I don't know whether I would or not. YMMV.


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