Sliding something onto the nose or ogive of the bullet hoping that it will stop at a point of consistency better than inconsistent meplats of many bullets has turned into a marketing gimmick that misleads shooters into thinking they can use this info to chamber a rifle to exact specifications.
You said "chamber a rifle", do you mean the actual act of cutting a chamber? What else am I going to use other than a reference bullet marked for minor diameter to spec a reamer? I'm not saying I can dial in a combination to 0.001", but 0.010-0.020" is reasonable even with a reamer's actual throat length vs print being pretty much a WAG for the first cut. If a reamer is good, it's good, and it gives you a useful range to work in. A one piece reamer is better (IMO) than using a separate throating reamer for most applications.
To your point, yes the ogive is the entire nose radius of the bullet, but at a point along that bullet minor bore diameter can be measured. I don't use a CBTO comparator for this, I set a caliper at the right size and use it to scribe a circular line at the correct point along the ogive. Same as measuring anything else - measure enough bullets enough times to get a measurement accurate enough for the intended use, then use it. Variances in bullets doesn't mean I'm going to stop measuring to make sure I don't short throat a chamber.
There are dimensional errors in throat cuts on the reamer side also (significantly worse in absolute measurements than on the bullet side, but more consistent since there's only one chamber per gun, SD and ES of 0.000" so to speak
).
Then marketed as a must have tool for the loading bench that most people have no use for.
No use for in the sense that it can be done in other ways maybe, but it's a simplification and combination of several concepts into a single tool that if used as designed for one specific job works fine.
There's a tolerance stack at play with all this:
- Bullet ogive consistency - usually varies +/-.001" in decent lots, more like +/-0.0005" in a good lot of bullets. Can be sorted out if you want. (see Litz's Vol III)
- Roundness of the hole in the comparator (Hornady is bad at this)
- Correct size of the hole in the comparator
- Shoulder dimensions of case used for measurement versus chamber
- Roundness and sizing of the seating stem interacts with the ogive variances during seating
- Correct throat dimensions on a reamer (this is the biggest variable, but only matters once per barrel)
- Here's one a lot of people don't think bout at all - roundness and straightness of the bore, and consistency in rifling lands heights vs the grooves. How many here have ever pin gauged an entire bore?
- ~1.7 million other things make very very small impacts, but in the end don't matter or aren't measurable
The tool still has a practical purpose, but it's more limited in scope than many people seem to use it for. This entire discussion is why I don't typically do super duper fine seating depths tests for many rifles, it's a very big task to reduce the tolerance stack to the point that you can get enough meaningful results to matter. It's one big set of relative measurements (made simpler with a cheap tool), so if it shoots fine - LEAVE IT!
