long guns wouldn't be included in this law. Only hand guns and rifles that are already illegal.
If that is the case, then the whole thing is grandstanding. Federal law already requires the purchaser of a handgun or handgun ammo to be 21. What federal law doesn't do is infringe on the possession of a handgun by persons under 21. You can't purchase but it is legal to possess under federal law. Illegal rifles applies to everyone. There are special cases of short barreled rifles that are technically illegal unless you get BATFE approval and pay the $200 NFA tax.
If this passes, you won't be able to be an armed police officer until you are over age 21.