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March 21, 2006

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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.



You can tell Kenny that instead of torching those actions he can just go ahead and send them my way. Also the 338 magnums article reminded me, I was just told by some really intelligent guy that the .338 Win. Mag is Winchesters new answer to the .338 Lapua.


Well this is my guess, and it's only a guess, but I suspect that it is an issue of over-lapping tolerances.

Think about it this way. Every process and application has its' respective tolerances. The raw materials used to form the steel fall into an acceptable and unacceptable range; however, even the acceptable materials have a tolerance range. The same goes for the completed steel product. In addition, the tools used to machine the steel are accurate only within say +/- 0.0001 of an inch.

If you combine all of these tolerances, and then take into account the statistical certitude that every so often all the components that fall at the unacceptable end of the spectrum are going to end up in the same unit. You begin to see how such differences are possible.

Finally, you also have to consider that we humans are incapable of replicating the same result each and every time we actuate a particular process.

Taken collectively, these factors might explain the phenomenon.


Steel is interesting stuff. By comparison to most metals it has a personality that is a lot more like wood. It has grain, varying temper, memory, varying hardness, and and records in it molecular structure, every temperature varience and impact it is exposed to since being poured. This affects the way it expands and contract with temperature changes, and the way it rings,its resonance and even the direction of oscilatory movement in resonance. No two pieces, even within the same batch are exactly alike in every mynute detail. It should be expected that every once in a while everything would add up wrong and you would just get a finished barrel that simply won't do what you might expect it to do.

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