Interesting question. You could further refine it to what percentage is "scientifically" able to be used for an ID to a specific firearm, and what percentage is usable to influence a jury?
I suspect a good portion of the "identification" performed is not to a specific firearm, but to narrow the possibilities down to an already suspected firearm: caliber, brand (e.g., distinctive Glock firing pin), rifling pattern (# lands/grooves, twist rate).
My sister was trying to convince me that at a local trial a "firearms expert" testified that a recovered bullet could tell whether the firearm was fired accidentally or on purpose?!?! Whether true or not, the "ballistic evidence" is used to influence a jury to somebody's benefit and another's detriment.
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