If your AR fits the legal definition of an assault rifle (or of an assault weapon, as in the infamous AWB), you'd be pretty much hosed, I think. As far as I know, showing that a "defamatory" statement is true is an iron-clad defense against an accusation of libel. The burden would be on you to prove that your AR wasn't
an assault rifle/weapon -- which would be difficult, given all the quibbling over exactly what they are. (and I suspect that the distinction between an "assault rifle" and an "assault weapon" isn't one that's going to cut much ice with the average judge or jury...
(That truth-of-the-accusation thing was what did in Oscar Wilde's lawsuit against Lord Whatsit, for example -- it didn't matter that being called a practitioner of the "infamous crime against nature" was, at the time, highly derogatory. If true, then not libelous.)