Isn’t the first thing to do in such cases is to check who in his primary and secondary circles has a licensed firearm? I am not detective, but that makes sense to me.
Also FWIW in MA firearms aren't licensed, rather their owners are. And ownership isn't registered, rather many types of transfers are supposed to be recorded.
Granted, even if you find such matches, you still have to prove motive and opportunity.
You'd also be excluding everybody who illegally has a firearm or knife or whatever the murder weapon is.
I think there’s also the issue that you’re more likely to be murdered by someone you know than a random person. At the very least, matching bullet marks from shots fired from his associates guns to any casings found at the scene is just due diligence.
The most rapidly increasing (although still small in absolute numbers) class of gun associated with crime today are the 3D-printed variations:
Thousands of guns are found at crime scenes. What do they tell us?
* https://www.npr.org/2025/12/17/nx-s1-5641154/crime-guns-data...
So, yes, if this a crime of passion, a dispute between aquaintainces that escalated badly then there's a good chance the gun used has a history of ownership and registration.
If this is a crime related to home invasion gone badly then it's more likely to be a gun that fell off the radar some time past.