Follow the logic!
Michael Mathias asks: "Is there any idiot wielding an illegal gun Owen Robinson won't defend?" Claims Robinson: "Of course, the obliterated serial number raises an eyebrow, but it isn’t illegal to scratch up your own gun and the ownership of the gun isn't in question."
Yes, it is illegal (see 18 U.S.C. §922(k)) to alter your own gun's serial number if that gun has at any time been shipped or transported in foreign or interstate commerce (likely in this case, as I don't believe there are any manufacturers of .357 caliber revolvers in Wisconsin).
Jack Lord only knows how Robinson is aware unquestionably of the firearm's ownership based on the facts presented in the Journal-Sentinel article, but ownership and possession aren't the same thing. One might very easily possess a firearm owned by another party.
Where does he imagine the concept of "theft" originated.