The die doesn't seat off the tip, it seats off the ogive.
This is why people gauge to ogive from base and sort accordingly.
Bullets are made in several dies, none will be exactly the same.
Don't know where you got your info, but, take the seating stem out and place a bullet in it, it doesn't touch the tip.
Cheers.
FWIW - the seating die stem doesn't contact the ogive at the same point where the caliper insert contacts the ogive. Rather, it contacts much farther out on the ogive toward the meplat (
red arrow below). The caliper inserts typically used to measure CBTO typically contact the ogive fairly close to the ogive/bearing surface junction, usually very near the point where the bullet ogive first touches the rifling (
two blue arrows below). Bullet variance between the two points where the seating die stem and the caliper insert contact the ogive is one of the things that causes seating depth inconsistency (
green double arrow below).
Sorting bullets by BTO will not help with a seating depth issue caused by bullet ogive variance between the points where the seating die stem and the caliper insert contact the bullet (
green double arrow below). This is because the BTO dimension is completely
outside the two points where the variance occurs; i.e. - the seating die stem and the caliper insert contacts on the ogive (
red and blue arrows below). BTO is measured from where one jaw of the caliper seats on the bullet base to where the caliper insert seats at (or near) the ogive/bearing surface transition (
blue arrows). Sorting bullets by BTO will ensure you have a consistent amount of bullet shank/boattail seated down in the neck/case, but it will not improve seating depth inconsistency caused by ogive variance between the caliper insert and seating die stem contact points. Frankly, the BTO dimension doesn't mean much from a practical viewpoint unless length variance in this region is fairly large. This is easy to demonstrate by simply measuring velocity during a seating depth test. What you will find is that you can move a jumped bullet quite a bit in either direction (~.010") without affecting velocity by an amount you can actually measure reliably with a typical chronograph. If bullet BTO measurements from a given lot aren't varying by more than this amount, sorting bullets by BTO won't accomplish much more than perhaps creating some peace of mind.
On the other hand, changing seating depth as measured from the
blue contact point(s) illustrated below, even by as little as a few thousandths, can have a huge impact on precision. The OP's question (from 5 and a half years ago) was simply asking why the seating die stem doesn't contact the bullet ogive at the same point (or very close) as the caliper insert does (~ where the bullet ogive first contacts the rifling). Although not a direct answer, a good approach is simply to sort bullets by ogive using a tool such as Bob Green's Comparator (or similar). This tool allows you to sort/group bullets so as to improve uniformity of the distance indicated below by the
green double arrow. Perhaps someone with a stronger mechanical engineering background than I possess can explain why current seating die stem technology doesn't work this way. A couple of technical reasons it might be difficult seem pretty obvious to me, but there there may be other reasons as well. Even though this thread is very old, there still seems to be a lot of confusion about what the bullet dimensions we measure actually are, and what they mean. I feel like this cartoon may be helpful to some, and was therefore worth posting.
