Wisconsin 941.28 prohibits short barreled shotguns and rifles. Therefore, according to the plain text of the Wisconsin statute it does not apply to Mr. Rittenhouse since the firearm he had is not a short barreled rifle.
The other sections cited, 29.304 and 29.593 are not relevant to Mr. Rittenhouse. 29.304 and 29.593 set age limits and other requirements for hunting.
It is hard to understand why the DA chose to even make the firearms charge, unless it was to try to mislead the jury.
I’ll go with “mislead”. Common jury will not intuit the difference between “minor illegally possessing a firearm” and “firearm as limited to NFA short-barreled firearms”.
Took me rather a while to realize NY defines a rifle as not a firearm, unless it is illegally “short barreled” - and that was as a well-trained owner spending lots of time perusing NY gun laws.