I couldn't get the full article, but am chalking this up to ignorance on the parts of both the cops, who seem to routinely dramatize their comments to reporters, who routinely,ignorantly, uninquisitively, and unquestioningly, regurgitate what they've been told. The guy, as Gatorfarmer said, probably had some commonly available cheap FMJ 7.62x39mm AK ammo lying around, contributing to what is usually breathlessly characterized as an "arsenal".
By the way, why has no one questioned the premise of the search, and the accuracy of the headline, which maybe should have been: "On the flimsiest of evidence, much of it hearsay, local police bust fireams enthusiast, discovering cheap ammo, paltry gun collection, and an insignificant amount of contraband cannabis."