I have heard of it being bunkum, but not quite of the same ~caliber~ as fiber matching and bite-mark matching.
It can, at best, match ammo from the same machine-manufactured batch to a factory-stock model of gun. It automatically fails on hand-load cartridges, or ammo that the analyst cannot readily source.
When there is a match, the prosecutor might say "this bullet was fired from this gun", but the science says "a bullet similar to the one in evidence was marked and deformed in a similar fashion, when fired by the weapon in evidence (just as it would with every other factory-stock copy of this model of gun)".
It's like matching a nail to the hammer that drove it. If the test nail is already different from the reference nail, you cannot match it. If you have two or more identical hammers, you can't tell which one drove the nail. The hammer factory made thousands of identical hammers.
In most cases, the tests are likely saying "the most commonly sold type of ammunition was fired from the most popular model of gun of that caliber". It's the sort of thing that should probably be used exclusively as exculpatory evidence. I.e. this model of gun always leaves distinctive markings, such as from an off-center diamond-shaped firing pin, or from an uncommon rifling twist, that were not found on the bullets in evidence, so the gun in evidence could not have fired them. Otherwise, a gun substantially similar, with all the same machine-made parts, could leave the same marks, so you could never know for sure which one fired the bullets.
The only time it would be useful to convict is if someone used a unique, hand-made, gunsmithed gun to fire factory-made ammunition. And the number of (violent) crimes committed with that type of display-piece gun approaches zero, because they tend to be both expensive and less fit for use as a weapon than cheaper factory-built guns.
It can, at best, match ammo from the same machine-manufactured batch to a factory-stock model of gun. It automatically fails on hand-load cartridges, or ammo that the analyst cannot readily source.
When there is a match, the prosecutor might say "this bullet was fired from this gun", but the science says "a bullet similar to the one in evidence was marked and deformed in a similar fashion, when fired by the weapon in evidence (just as it would with every other factory-stock copy of this model of gun)".
It's like matching a nail to the hammer that drove it. If the test nail is already different from the reference nail, you cannot match it. If you have two or more identical hammers, you can't tell which one drove the nail. The hammer factory made thousands of identical hammers.
In most cases, the tests are likely saying "the most commonly sold type of ammunition was fired from the most popular model of gun of that caliber". It's the sort of thing that should probably be used exclusively as exculpatory evidence. I.e. this model of gun always leaves distinctive markings, such as from an off-center diamond-shaped firing pin, or from an uncommon rifling twist, that were not found on the bullets in evidence, so the gun in evidence could not have fired them. Otherwise, a gun substantially similar, with all the same machine-made parts, could leave the same marks, so you could never know for sure which one fired the bullets.
The only time it would be useful to convict is if someone used a unique, hand-made, gunsmithed gun to fire factory-made ammunition. And the number of (violent) crimes committed with that type of display-piece gun approaches zero, because they tend to be both expensive and less fit for use as a weapon than cheaper factory-built guns.