Dominic Black is on trial for making a "straw man" purchase of the rifle - in other words, for buying a gun for someone who otherwise could not legally buy (as opposed to legally carry) the gun. Now, it would have been perfectly legal for Black to buy the gun for Kyle as a gift and give it to him on his 18th birthday, but it appears that Kyle gave Black the money to buy the gun and Black was to hold the gun for Kyle until he was 18. That is an illegal "straw man" purchase and is actually a federal crime. (I used to have my FFL, so I remember this provision quite well.) So I would imagine that the rifle in question is still evidence in Black's case. If he is convicted, I doubt that Kyle would get the rifle, since he is not the legal owner.
The AR-15 that Kyle Rittenhouse used to defend himself would go for top dollar at auction, especially if done by a lottery. How many would buy tickets for $100 for a chance to own a rifle that's such a part of history. Even virtue-whiners would by ticket for a chance to destroy it. How much would a colonial rifle from the "shot heard around the world" go for if it could be matched by serial number as this rifle can be? Kyle shifted the course of the Democrat Riots of 2020 with this one.
No, you are incorrect. Kyle is not charged with an federal gun crime, and for good reason.
For a "straw man" purchase to be illegal, the gun has to be substantially transferred to the other person. The Smith & Wesson M15 was never substantially transferred to Kyle.
He is charged with two counts of the State crime in Wisconsin Statutes 948.60 2(a). But the judge dismissed the charge against Kyle (because of the exception in 948.60 3(c). The same exception exists for Dominick.