Previously, there was a discussion on the Stoney Point comparator. Another use is sorting bullets. Its common when seating bullets to find variations in both the LOL and COL lengths. It your trying to get every bit of accuracy out of 'ol Betsy or your favorite varmint rig, sorting bullets may help.
Lets say you want to seat .010" off the lands, so you measure the depth of the chamber with a modified case with your favorite bullet. You take that reading, seat a bullet 10 off and load 100 rounds with that setting. All the same right? Well, mabye not. Two things may affect that measurement. First is the casehead runout and second, the ogive placement on the bullet. All bullets are not created equal even from the most respected makers. Mass produced bullets can vary quite a bit and short of buying only the best quality, like Fowler's,BIBs,etc, the average guy picks up Hornady,Nosler,Sierra and so on. Contrary to what is said, bullets from different machines are (sometimes) mixed in the same box and lot. If one machine is out of adjustment or the dies worn, variations in bullet dimensions will occure. To make the matters worse, setting 10 machines up exactly the same is almost impossible. This may be one of the reasons we get flyers or shoot good groups one day and shotgun patterns the next.
Using the tool to check and sort bullets from the base to ogive may help keeping the shots in one spot.
In the picture below are a group of 6mm bullets from a well known manufacturer,same box, same lot. The variation,from a low of .436" up to one at .445", .007" difference in the bullet bearing surface.
Recently I received 400 68 grain flat based 6mm "Custom" swaged bullets from someone trying to make accurate BR slugs. The base to ogive varied from .351" to .382". I think his process needs a bit more work.
I like experimenting, so most of my seaters have been modified to contact close on the bullet ogive to help eliminate COL lengths due to variations forward of the ogive.
Well, I just wanted to pass this info on. I'm sure some already know this stuff, but for those that didn't, mabye it will be of some help.
By the way, a even more accurate method is to use two comparators for boat tail bullets, one on the ogive and the other on the base.