That's fair. So why exactly do you think that 18-22 year olds should be denied constitutional rights, yet still be required to register for the draft? What other constitutional rights can they be denied? There is a well established federal legislative history of disallowing FFL holders from selling certain guns, handguns specifically, to them, but still allowing private transfers. Now, this is relevant because that was allowed because it was not believed that it was constitutional to completely deny them the right to acquire a handgun. However with the expansion of the commerce clause, it was believed to be justified to deny them FFL transfers. Getting further into legislative history, when the NFA of 1934 was being debated in congress, they originally wanted to include handguns in the list of firearms that would be subject to the law. However, they felt this would not pass constitutional muster and dropped them so that it would not risk SCOTUS striking the whole thing down. At the time, the commerce clause had not been expanded, so it was passed under Congress's authority to levy taxes. Of course, this was before the second amendment had been incorporated under the fourteenth amendment to the states, something which has not been challenged given that that only happened in 2010. Given the fate of other laws that tax civil rights which have been incorporated under the fourteenth amendment, it could very well be that the transfer tax goes bye bye as well. Anyhow, that's a lot of words to say that there is a long legislative history of allowing certain restrictions, but not outright bans, on certain firearms for certain people who have not been denied their rights through due process. There have been lower court cases, and the results are fairly predictable if you follow that stuff, but there is no real established law. The real question is do you want to establish it with the current court?
Maybe the better tactic would be to disallow those transfers through FFL's while still allowing private transfers. You could even throw in a universal background check system for all private transfers that would be akin to the one Senator Coburn proposed in 2013, except run by the state instead of the federal government. If you have to, make it optional for five years to test it out and get enough votes to pass and then see what the results are before trying to make it mandatory. I would suspect that a system like that would be used for the vast majority of private transfers between strangers.
Also, write better bills. Bozo's Mini-14's only saving grace, beyond it's terrible accuracy, is that it is to expensive for angsty teen incels. But something like this:
Or this:
Is not likely at all to be used in a mass shooting event. Yet the bill discussed previously would ban them. And probably also an freaking Ruger 10/22, which is the most popular and practical .22 long rifle guns made in the modern era.
For real, write better bills that are actually well thought out instead of coming up with some crap that couldn't even pass a cursory review by someone knowledgeable on the subject and is likely to get struck down in the courts.