There's no way libel laws could conceivably apply here: libel (and slander, which is basically the spoken version of libel) is a tort committed against a person
(or corporate entity, but don't get me started on the whole "corporate personhood" thing).
Don't see how you're gonna argue that a gun is a person.
And anyway, it's the wronged individual (technically known as the "plaintiff") who has to bring suit against the party alleged to have libeled him/her, so unless you think you're an SKS or whatever, you have no standing. (And if you do think that, your problems may go deeper than this issue...
Edited to add:
As to firearms companies bringing such suits, they'd have a hard time arguing that a term they invented themselves is libelous. From a rather good, balanced article
in today's NYT:
"...[T]he term “assault rifle” was expanded and broadened when gun manufacturers began to sell firearms modeled after the new military rifles to civilians. In 1984, Guns & Ammo advertised a book called “Assault Firearms,” which it said was “full of the hottest hardware available today.”
“The popularly held idea that the term ‘assault weapon’ originated with antigun activists, media or politicians is wrong,” Mr. Peterson wrote. “The term was first adopted by the manufacturers, wholesalers, importers and dealers in the American firearms industry to stimulate sales of certain firearms that did not have an appearance that was familiar to many firearm owners. The manufacturers and gun writers of the day needed a catchy name to identify this new type of gun.”