I read one comment that suggested you carry the ATF determination letter on the firearm, in the grip, etc, so it becomes part of the evidence. IANAL
ATF Unhinged: Prosecutions Made Up Out of Whole Cloth – You Might Be Next…
Posted on October 28, 2018 by Joshua Prince, Esq.
Since the Law Vegas shooting and what some contend has been a takeover by Department of Justice (which in some contexts appears to be accurate), ATF has begun internally reversing prior determinations and making up new interpretations of law, in the absence of informing the Firearms Industry or the public of these reversals and/or new interpretations. More disconcerting, this mentality has now seemingly infected certain U.S. Attorney Offices, which is alarming to say the least.
Although I cannot disclose all of the occasions where ATF has recently reversed its prior determinations or devised a new interpretation of the law or regulation, I can disclose a recent prosecution, of a veteran, where ATF devised a new interpretation out of whole cloth and was successful in convincing the U.S. Attorney’s Office for the Northern District of Ohio to prosecute. The case is U.S. v. Wright, 3:18-CR-162 and it should have the entire Firearms Community alarmed.
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More at link: https://blog.princelaw.com/2018/10/28/atf-unhinged-prosecutions-made-up-out-of-whole-cloth-you-might-be-next/