One of my friends recently told me they sort bullets by both weight and length of their bearing surface (the part of the bullet that comes in contact with the barrel, illustrated below). I hadn’t thought of that before, but you could see how variations in the bearing surface length obviously impact the amount of barrel friction on the bullet and therefore cause variations in muzzle velocity.
For virtually all of my precision shooting I use Berger bullets, and I wondered if there was even any measurable variation in their bearing surface because their quality control is so much tighter than other bullet makers. So I measured the bearing surface on over 150 bullets, and here are the results I found.
Bullet: Berger 7mm 168gr Hunting VLD (Lot #4375, Purchased Oct 2012)
Standard Deviation: 0.00073″
There is a measurable difference … larger than I originally expected. You can decide whether this is significant or not. I know people who claim that it is, and although I am currently still sorting bullets by bearing length I’m not sure if it is statistically significant. Essentially I’ve found a standard deviation of 0.00073″, which is 0.1% of the bearing surface length. I wouldn’t think 1/10th of a percent could have enough of an effect on barrel friction to result in any measurable effect on muzzle velocity … but I can’t prove that, so the decision is up to you. I would expect bullets that weren’t match-grade may show higher variance in bearing surface length.
How To Measure True Bullet Bearing Surface Length
Some people measure bearing surface differently. A lot of people just measure from the bullet base to ogive, which is essentially the bearing surface if it is a flat base bullet. However, with boat tail bullets that isn’t the bearing surface. You actually have to attach two bullet comparators to a caliper so that it measures from the start of the bearing surface on the rear/bottom of the bullet, instead of the base of the bullet.
So here is the setup I use to measure bullet bearing surface length. It is essentially two Hornady Lock-N-Load Bullet Comparators of the same caliber attached to a caliper. You need to ensure they are precisely aligned for repeatable measurements.