How about making laws that stop, or at limit, the opportunity for the crime before it happens, instead of punishment after the event?
There is a very simple law that could be passed that would be constitutional and would also have a big impact on reducing the number of guns in the hands of felons and gang members and people who are mentally unstable. It would also alert police when someone who has a gun legally is behaving in an alarming way (buying up many guns, buying thousands of rounds of ammo, buying body armor, etc). All you need to do is require liability insurance for anyone who owns a gun. For most gun owners the liability would be very low. Then whenever you buy a gun, or ammo, or body armor or anything else you must show proof of insurance and the insurance company will be alerted. Also to get a break on the insurance you give them access to medical records including mental health.
Here is how that would reduce guns in the hands of felons.
1. Felons are not allowed to buy guns so they use mules who have no criminal record who then claim the guns were "stolen" from him. However, if he had to have insurance to buy the guns the insurance company is going to balk when he wants to buy 25 AK's. Also, when the gun is insured it will have a digital record identifying the bullet and shell casing fired from that gun. As a result gun dealers who have an inordinate number of guns showing up in gang murders can be shut down. It is estimated that 1% of gun dealers are the ones supplying the guns to gangs, so shutting them down cuts off their pipeline to guns.
2. If you catch a gang member with a gun you have a second violation (no insurance) that you can use to hold the criminal for 24 hours or even a few days. This gives you plenty of time to run the digital record on that gun against any and all crimes nationwide. This means that criminals will have to throw away a gun after each crime and if it is very hard to get another one, they will be very reluctant to do that, as a result you can start catching them with the gun and solving many "cold cases".
3. Insurance companies have the best mathematicians who use sophisticated algorithms, AI and super computers. As a result as soon as something that triggers a red flag occurs the insurance company would be alerted and would have a financial incentive to intervene. For example, a man gets divorced, or his children are taken from him, or he just bought 1,000 rounds of ammo, or he just got fired, or he has begun meeting with a therapist, etc.
The constitution says "well regulated militia" so requiring liability insurance could be viewed as "well regulated".