Then why write "This section applies only to a person under 18 years of age who possesses or is armed with a rifle or a shotgun if the person is in violation of s. 941.28 or is not in compliance with ss. 29.304 and 29.593." into the law? 29.304 and 29.593 only apply to people under 16, and 941.28 deals with a prohibition on short-barreled rifles and short-barreled shotguns. Which is why the length of the barrel matters - it not being short barreled means it's not in violation of 941.28, and he's over 16 which means 29.304 and 29.593 also don't apply.The worst part is it's not a technicality.
https://docs.legis.wisconsin.gov/misc/lc/information_memos/2018/im_2018_02
The Wisconsin legislature clarified in 2018 precisely what the law was meant to do in this situation.
Why have a specific exception for rifles and shotguns in the hands of 17 year olds if you don't want an exception for rifles and shotguns the hands of 17 year olds?