NIBIN is the national database where bullets recovered from crime scenes are entered. Sometimes you identify gang related shootings years apart because a gun hasn't been used in that time.

Touch DNA is recoverable on a consistent basis, especially on rough surfaces like pistol grips.

Serial number obliteration can be done, and the exact method isn't relevant.

I have 8.5 years of crime lab experience.

8.5 years working in a crime lab.

If you have two bullets from two different scenes, you can match them in a comparison to say the same gun was used in both crimes.

If you have the gun, you can swab it for DNA and run it against the criminal database.

Serial numbers can be effectively obliterated if you know how.