You know when they do ballistics testing on a gun and somehow determine a bullet was fired from one specific gun and not another? How do they know it had to have come from that gun and not the one with a serial number 1 higher or lower? Aren't hundreds or thousands of barrels bored out by the exact same machine and would be pretty much the same? I would think since they rely on one person looking at two bullet or bullet fragments and not some computer aided matching, wouldn't it be open for a lot of human error as well? How can there be such a huge difference between two barrels bored out minutes apart by the same machine?