Not correct. Judge Schroeder even alluded to this in the initial jury instructions.
People 16 and 17 years old have never been forbidden from possessing and using rifles and shotguns in Wisconsin law.
The defense has moved to dismiss the weapons charge. Judge Schroeder has not made a ruling on it yet.
People under 18 are forbidden from possessing "dangerous weapons" which is a very specific definition under Wisconsin law. Rifles and Shotguns are exempted from the law.
The exemption isn’t apparent from the definition at https://docs.legis.wisconsin.gov/statutes/statutes/939/i/22/10
I know some states do separate “dangerous weapon” from “firearm” etc; not versed in Wisconsin law so don’t see the division apply (yet).
That said, the point is easily refuted by “doctrine of competing harms”, to wit: doing something illegal is not punishable if following the law would cause greater harm. Absent the rifle, all else same, Kyle would presumably be dead or gravely injured - so possession of the rifle is an “affirmative defense” to wit “yes I broke that law, and this is why doing so is allowed...”.