An AK-47 is not relevent to a murder investigation.
There are thousands of them in the U.S. and owned by innocents. It would be unreasonable to come after all AK-47 owners because one was used in a crime. Rather, it would be up to the cops to find other facts about the weapon, perhaps how much more metallic it looked than your standard AK, and in which spots. Wear marks can be just as revealing as serial numbers.
You’re describing due process. That’s a feature. An AK-47 being metallic may be self evident. That doesn’t make it relevant to a murder investigation.