Two barrels of the same nominal length for the same caliber, and same model are not truly identical. They are mostly hammer forged now, I think, which means that a blank is heated, softened, and hammered around a die. But there can be miniscule differences in the metallurgy of 2 barrels, if there are 2 different dies used, they won't be identical in size and shape throughout their entire length, and when the barrels are heat-treated to harden them, they won't come out with the same identical hardness throughout. Also, the bullets are cast or swaged, and there may be slight differences in weight due to slight differences in the lead/tin/antimony/copper alloy in 2 bullets from the same batch. The tolerances used in controlling the barrels and bullets are in the order of 1/10,000th of an inch, but they are not expected to be exactly identical in every respect. Also the powder charges are weighed to 1/10th of a grain, so there may be a few 100ths of a grain difference. Often the grooves in a barrel are not perfectly smooth, and therefore invoke small differences in the friction the bullets encounter when they are shot.
Last edited by AIKIJUTSU; 04-12-2013 at 08:42 PM.