Ballistic fingerprinting

Various people have written on this in the past, here's my way of putting it. I think many of the folks who have read my words here are familiar with it, but for anyone who comes by who isn't, here it is.

Fingerprinting works because your prints don't change. You can become scarred on your hands and that simply adds to the ID marks; what remains of your original prints has the scars added to them. Basically, the prints you are born with do not change. Ballistic 'fingerprinting' is based on this idea.

First, when a bullet travels down the barrel of a rifle or handgun, the rifling- the spiral grooves in the bore- grip the bullet to spin it. This engraves the marks into the metal of the bullet, and unless it's completely smashed/mangled by the impact when it hits, these markings can be seen.
Second, the cartridge case is expanded against the walls of the chamber by the pressure of the shot, and also pushed back against the breech face. This causes the shape of the chamber and breech, any scratches, etc., to leave marks. And in semi-or automatic firearms, the extractor- the hook that pulls the case out of the chamber- can also leave marks.
So the idea is that if you save a fired bullet and cartridge case from a gun, then index the marks, then you could- supposedly- take a bullet or case found at a crime scene and run it through a computer and find what gun it matches up to.

However....

This ignores a number of things. Take a brand-new gun and fire it; save the samples; and then, after someone has fired several hundred rounds through it, compare them again. Good chance you will not be able to say, positively, "this bullet/case came from this particular firearm", because these marks change. You don't have to make a deliberate effort to change them, the normal wear of firing does that for you. Cleaning, especially with some abrasive cleaning/polishing compounds, does it. And, as the California AG's study showed, simply firing a different brand of ammo can cause the marks to be different. Different bullet design/materials, slight differences in case material/design, can make a match uncertain ranging to negative.

This is not the same as finding a gun in the posession of a suspect and comparing it with bullets/cases found at the crime scene. In this case it's probably the same ammo, and right after the shooting. You have a very good chance of making a match then. Otherwise...

One of the other things not considered is that in modern manufacturing, barrels are made one after another on the same equipment; as they come off the line, they are virtually identical and marks may not be distinguishable between them. Which means that any normal wear makes immediate changes to the markings left on a bullet. Same thing for the cartridge cases; the chamber in a semi-auto is part of the barrel, so the same holds for the markings on the case. In revolvers you might well have more variation in that the chambers are cut in the cylinder that holds the cartridges, but again they are made one after another, on the same equipment.

The slide of a semi-auto and the frame of a revolver, again are made one after another. There might be more variation here, but again normal wear changes the marks.

Now you get into another factor; deliberate changes. Doesn't have to have a thing to do with trying to change the marks.
A shooter buys an after-market barrel for this semi-auto either for a feature like a compensator on the muzzle or to change cartridges- and yes, some semi-autos can be changed to a different cartridge just by changing the barrel.
A shooter wants to improve the barrel, so he uses one of the many methods to polish the bore.
The breech-face gets polished for smoother operation.
The extractor is polished or replaced, again for smoother functioning.
After-market firing pin for faster lock time.
Any of these will change the marks left on the bullet/case, and the sample the police have now means nothing- except that they have to store it, which takes up space and money.

And the above methods can be used to deliberately change the markings, and are used. I have no idea how many people I've heard of who, first thing they do with a new firearm, is polish the bore, etc., just to change the marks. Their way of telling the authorities 'screw you'. These are honest citazens who just don't like the government keeping records on their property.
Another reason many dislike it is that it is a de facto method of gun registration. And again, unless you are violating the law in your use of something, it is none of the governments' business what you own.

Ballistic fingerprinting as I see it. And apparently the honest people in law enforcement see it the same, as even the Peoples Republic of Maryland is asking for changes in law so they can drop this, and put the money and people into improving their DNA database of criminals, which has the advantage of actually working.