Why is it that two consecutive rifles off an assembly line will shoot so differently?

Here’s one for you to ponder: Compared to most other machines, rifles are very simple mechanism—very few parts, and those are uncomplicated. Then why is it that two consecutive rifles off an assembly line will shoot so differently? Or to take it a step further, why is it that two custom barrels, made with the most exacting precision, will shoot differently? I mean, I know one barrel maker who does his own spectrographic analysis of each load of barrel blanks he gets, just to make sure the steel mill that produced them isn’t slipping something by him, and even his barrels don’t shoot alike.

Kenny Jarrett, the South Carolina gunmaker who specializes in sub-minute (and usually sub-half-minute) rifles of all sorts, from prairie dog rifles to buffalo rifles, once told me that when he was using factory actions as the basis for his creations, he would get two or so a year that simply would not shoot. I mean, whatever they did to the rifle, and they did everything, it simply would not shoot well. All they could do was cut the receiver in half, throw it on the scrap heap, and start again.

Kenny had no idea why this should be, and neither do I. If you have any thoughts I’d be interested in hearing them.