Originally posted by MichaeltheGreat
The reasons can generally be lumped under "waste of time and money" and "potential for abuse"
In the first category, it's a waste because the marking characteristics of barrels change with excessive wear (put 5000 rounds of FMJ through a rifle barrel in a fairly short period of time, and you have different markings). Some times of ammo (some hollow points, Glaser safety slugs and wadcutters) fragment so badly ballistic markings are unrecoverable. Also, good ol' .22 long rifle tends to be very hard to trace. It's also easy as hell to damage the surface of the lands and grooves within the barrel.
On the abuse side, I think there's been enough reversals of DP convictions, police scandals, crime lab scandals, etc., where overzealous cops, prosecutors or lab technicians have already reached a conclusion as to who is guilty. Most criminal defendants don't have access to defense experts and independent labs, so giving police and prosecutors a tool that points to a name and a particular weapon type is a little too powerful. Just take a dumb as a rock jury, claim that there's a higher probability ballistic match than there really is (who's gonna know, and hell, we all know the ******* is guilty?), and point out that iron-clad "fingerprint" that Billy Bob bought a weapon just like this. (let's gloss over we can't prove he didn't sell it a couple years ago like he claims and we can't tie him to the crime scene
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The reasons can generally be lumped under "waste of time and money" and "potential for abuse"
In the first category, it's a waste because the marking characteristics of barrels change with excessive wear (put 5000 rounds of FMJ through a rifle barrel in a fairly short period of time, and you have different markings). Some times of ammo (some hollow points, Glaser safety slugs and wadcutters) fragment so badly ballistic markings are unrecoverable. Also, good ol' .22 long rifle tends to be very hard to trace. It's also easy as hell to damage the surface of the lands and grooves within the barrel.
On the abuse side, I think there's been enough reversals of DP convictions, police scandals, crime lab scandals, etc., where overzealous cops, prosecutors or lab technicians have already reached a conclusion as to who is guilty. Most criminal defendants don't have access to defense experts and independent labs, so giving police and prosecutors a tool that points to a name and a particular weapon type is a little too powerful. Just take a dumb as a rock jury, claim that there's a higher probability ballistic match than there really is (who's gonna know, and hell, we all know the ******* is guilty?), and point out that iron-clad "fingerprint" that Billy Bob bought a weapon just like this. (let's gloss over we can't prove he didn't sell it a couple years ago like he claims and we can't tie him to the crime scene

With regards to potential abuse, in theory the police could do this anytime they wanted. Heck, they could send cops out into the woods right now, find someone who uses a weapon similar to that used by the DC sniper, arrest the guy, fire off some rounds, then put them in the evidence bags as substitutes for the ones they collected at the scenes of the shooting. Wham Bam Thank You Mam, instant conviction!
If Billy Bob buys a weapon and ballistic fingerprinting is required it would behoove us all to institute registration similar to that used by the DMV. When Billy Bob sells his gun there will be a record of it.
If it was Billy Bobs van that the sniper was using, and Billy Bob sold it without bothering to change the registration, then some witness manages to copy the license plate number you can be sure that a swat team would be at his house within minutes.
None of your objections hold any real validity.
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