Well no civilian needs any AR platform, useless for hunting, overkill for self defense, (to be honest I’ve no idea how to solve that. No buyback would get all of them and confiscating isn’t going to work either), age needs to be 21 to purchase a firearm, it should have to be registered, background checks required no exceptions, and training should certainly be required as well. That’d be a nice start.
Here's some of my thoughts on these.
1) According to the FBI, ARs constitute an infinitesimally small fraction of firearms crime. While apparently frequently used in mass shootings, in truth they are usually abandoned by the shooter who ends up using a pistol or shotgun for the bulk of the work. They are widely used in hunting, especially small game, and are ideal for self defense because they don't over-penetrate (endangering other occupants of a home or apartment complex, for example). Moreover, an AR15 is functionally no different from, say, the Mini-14. What problems would banning the AR platform solve if similar 5.56 semi automatic rifles are exact drop-in replacements?
2) Why does the purchasing age need to be 21? At 18, you're a free adult - all your rights are unlocked. You can buy a car, buy a house, join the army, move anywhere you want, vote, become a police officer, etc. Under-21s constitute a small percentage of firearms crime and what percentage they do account for is mostly committed with guns stolen from relatives.
3) Registries have never been demonstrated to have any positive impact on crime rates (t. CDC, FBI). It's one of those things that sounds like it might be useful, but in practice, it isn't.
4) Background checks are already required on all sales of new firearms. They are not required for transfers between civilians - in fact, non-FFLs aren't
allowed to run background checks through NICS. I can get behind this one - I would feel a lot better if I at least had the option of running the check before selling a firearm to another private party.
5) Training required is a tough one. I'm cool with it in principle, but who pays for it? How do you know if someone's already had it? Schools used to have basic firearms handling courses as part of health/life skills classes, but that's no longer the case. Should we go back to that?