Mark you, I am not an armorer or ballistics expert, but it seems to me that if there was created a database of sample projectiles from all pistols and rifles, if I were a criminal the first thing I would do when I bought/stole/was given a firearm would be to try to change the marking characteristics of the rifle bore.
Seems to me that coating a single round with a little grease with abrasive in it and firing the weapon would change the marks left on subsequent bullets passing through the barrel enough to make matching with previous samples difficult to impossible. Yes, it might tend to wear out the barrel faster, and possibly decrease the accuracy of the weapon, but especially for the cheap Saturday Night Specials, which are only useful close up anyway, are cheap to replace, and probably would be disposed of after firing in the course of a crime, it would probably be worth it.