What happens to grandfathered items that are willed if they become illegal?

The assault rifles should be turned into - ask the police where to take them.  Antiques - there's no reason why you cannot keep those, unless you are a felon.  If you are a felon, then give them away or sell them.  You are not supposed to have them in your home.

They are either grandfathered or illegal.  If they are illegal, they are illegal, period.  Nothing to do with the will.  You can inherit the firearms, they are just illegal.  You then deal with the consequences of owning illegal firearms.

If they are a known and no longer legal, the estate will have to turn them over, if they can find them that is. The new owner, will not be able to legally register them according to whatever requirements were put in place.

The law prevails over a Will.  If something is deemed "illegal" for ANY reason it is required to be turned into whatever Government agency is in charge.  My uncle owns his rifle from Viet Nam (which I would guess he smuggled back in the USA, but that's another topic), took it to the Bureau of Firearms in our State, and they did whatever it take to modify it and change its class to "non working."

Yes, it is possible that inheritance will not be allowed. If a new law is passed, but someone's rights are grandfathered in, those rights generally do not pass to another person. Perhaps the person you have willed them to will have better sense than to want assault weapons anyway.