Cartridge base-to-ogive (CBTO) variance of up to a few thousandths is not uncommon. As has been mentioned, bullet length variance is most often the cause. A significant portion of bullet length variance will reside in the nose region, presumably due to the manufacturing process used to produce jacketed bullets. Sorting bullets base-to-ogive (BTO) will
not fix this issue because it is caused by length variance in the nose region. More specifically, it is caused by length variance between the caliper insert and seating die stem contact points, which lie outside the base-to-ogive measurement as shown in the cartoon below (
critical distance, in green).
A tool such as Bob Green's comparator sorts bullets based on this specific dimension and facilitates more uniform seating depth without having to constantly tweak the seating die micrometer. Alternatively, the seating die micrometer can be set such that the bullets having the
longest caliper insert-to-seating die stem contact measurement will be seated at the desired depth; those having a
shorter distance between those two critical points will be seated slightly longer (CBTO). In other words, some of the loaded rounds will have the desired CBTO, all others will be slightly longer. Those can be set aside and given another stroke with the seating die after adjusting the micrometer by the appropriate amount. Of course, this approach requires more time/effort and also necessitates that each loaded round be measured.
The reloader will have to decide how much CBTO variance is acceptable and whether it is worth the extra effort. I test seating depth in .003" increments and try to keep the CBTO of all my loaded rounds for matches within .001". For someone that isn't loading for competition or that tests seating depth in .005" (or larger) increments, trying to hold loaded rounds to .001" CBTO variance may be unnecessary.
For loaded rounds where the bullets have length variance in the base-to-ogive dimension (i.e. have not been sorted BTO) that have loaded with very uniform seating depth (CBTO), by definition there will be variance in the length of bullet shank in the neck and the effective case volume. This is solely due to the bullet BTO length variance and can be improved by sorting bullets BTO. However, in my hands, changing the seating depth for jumped (not jammed) bullets during seating depth testing by as much as .010" to .015" in either direction does not cause a
statistically significant change in measured average muzzle velocities, even though the effective internal case volume (and therefore pressure) is clearly altered by the different amount of bullet shank in the neck. In other words, my average velocities during seating depth testing over perhaps a .024" total seating depth range do not vary by more than the individual SDs. In contrast, changing seating depth by .025" to .030" (or more) may be enough cause detectable velocity variance. Clearly small differences in seating depth, or by analogy, bullet shank length (i.e. less than .010" to .015") are not usually sufficient to cause significant case volume/pressure/velocity changes. Again, the reloader will have to decide for themselves whether BTO sorting is worth the time and effort. It should be noted that the length of bullet shank in the neck can also affect neck friction and release of the bullet, in addition to the typically minor effects on case volume/pressure/velocity. It may be that the friction and bullet release are a more important reason to sort bullets BTO than the relatively minor changes in case volume/pressure/velocity caused by a few thousandths BTO variance.
