Ned Ludd
Silver $$ Contributor
In an ideal world, we would use a caliper insert made from a barrel stub that was cut with the same reamer used to chamber the rifle. However, not everyone is able and/or willing to do that for every rifle they own. So many use commercial caliper inserts that may not seat at exactly the same place on the bullet ogive as where the bullet ogive will first contact the lands. Typically, commercial caliper inserts will seat at a point on the bullet ogive fairly close to where it will first contact the lands, or perhaps slightly above, depending on the diameter of the hole in the caliper insert.
The real question is whether this potential difference in caliper insert contact points is really an issue in terms of seating depth consistency. I am of the mind that in many cases it is not. More specifically, it should not be an issue for shooters that use a measurement tool such as the Hornady OAL gauge (Stony Point Tool) to measure the distance to "touching" and are seating bullets off the lands. We use the measurement at "touching" as a reference point only. As long as the exact same caliper insert is used consistently, the measurements generated will be valid. I do not claim such measurements are necessarily 100% accurate, only that they can be reproduced consistently using the same tool/caliper insert. For example, depending on the tool used and the skill of the operator, it may be that for the measurement taken at "touching", the bullet may actually be seated .003" into the lands. Does this really matter as long as that specific measurement at "touching" can be reproduced consistently? I don't believe that it does in the specific context of loads with jumped bullets.
Once the reference measurement at "touching" has been taken, the CBTO dimensions (i.e. relative seating depth) of loaded rounds can be measured with very good accuracy. Further, those rounds will then be fired and the optimal seating depth chosen based on the targets. The CBTO measurement of the loaded rounds that exhibited optimal precision (i.e. optimal relative seating depth) can then be reproduced at will with very good accuracy in a manner that does not depend on the relationship between the seating die stem and comparator insert contact points. The fact that the actual amount of bullet jump may not be exactly what the user thinks it is no longer comes into play as the specific CBTO of loaded rounds can be reproduced at will.
In the example I used above, the CBTO measurement believed to be taken at "touching" was really taken with the bullet .003" into the lands. If the reloader finds that according to their measurements, a seating depth of .020" off the lands provides optimal precision. In reality, their bullet is only .017" off the lands because it was actually seated .003" into the lands rather than at "touching" when the measurement was taken. Nonetheless, that seating depth is where they observed optimal precision on the target, using loaded rounds for which it is quite easy to obtain an accurate and reproducible CBTO measurement for subsequent loadings. Within reason, slight variances in the caliper insert holes aren't a big deal as long as one uses the same caliper insert all the time for loading rounds of a given bullet diameter.
As a scientist by training, I initially found this idea of using a somewhat indirect reference measurement that may be precise but not necessarily accurate rather appalling. In fact, it drove me absolutely bonkers for a couple years when I first started reloading. Well, in fairness I should say that it drove me even more bonkers than I was before I started reloading, which again is a relative measurement; precise, but not necessarily accurate LOL. I eventually got to the point where it didn't bother me every single time I reloaded, and after a few more years, I hardly think about it at all. Of course, for anyone that simply cannot get over the notion of using a reference measurement that may be precise, but not necessarily accurate, I believe there are therapy groups.
The real question is whether this potential difference in caliper insert contact points is really an issue in terms of seating depth consistency. I am of the mind that in many cases it is not. More specifically, it should not be an issue for shooters that use a measurement tool such as the Hornady OAL gauge (Stony Point Tool) to measure the distance to "touching" and are seating bullets off the lands. We use the measurement at "touching" as a reference point only. As long as the exact same caliper insert is used consistently, the measurements generated will be valid. I do not claim such measurements are necessarily 100% accurate, only that they can be reproduced consistently using the same tool/caliper insert. For example, depending on the tool used and the skill of the operator, it may be that for the measurement taken at "touching", the bullet may actually be seated .003" into the lands. Does this really matter as long as that specific measurement at "touching" can be reproduced consistently? I don't believe that it does in the specific context of loads with jumped bullets.
Once the reference measurement at "touching" has been taken, the CBTO dimensions (i.e. relative seating depth) of loaded rounds can be measured with very good accuracy. Further, those rounds will then be fired and the optimal seating depth chosen based on the targets. The CBTO measurement of the loaded rounds that exhibited optimal precision (i.e. optimal relative seating depth) can then be reproduced at will with very good accuracy in a manner that does not depend on the relationship between the seating die stem and comparator insert contact points. The fact that the actual amount of bullet jump may not be exactly what the user thinks it is no longer comes into play as the specific CBTO of loaded rounds can be reproduced at will.
In the example I used above, the CBTO measurement believed to be taken at "touching" was really taken with the bullet .003" into the lands. If the reloader finds that according to their measurements, a seating depth of .020" off the lands provides optimal precision. In reality, their bullet is only .017" off the lands because it was actually seated .003" into the lands rather than at "touching" when the measurement was taken. Nonetheless, that seating depth is where they observed optimal precision on the target, using loaded rounds for which it is quite easy to obtain an accurate and reproducible CBTO measurement for subsequent loadings. Within reason, slight variances in the caliper insert holes aren't a big deal as long as one uses the same caliper insert all the time for loading rounds of a given bullet diameter.
As a scientist by training, I initially found this idea of using a somewhat indirect reference measurement that may be precise but not necessarily accurate rather appalling. In fact, it drove me absolutely bonkers for a couple years when I first started reloading. Well, in fairness I should say that it drove me even more bonkers than I was before I started reloading, which again is a relative measurement; precise, but not necessarily accurate LOL. I eventually got to the point where it didn't bother me every single time I reloaded, and after a few more years, I hardly think about it at all. Of course, for anyone that simply cannot get over the notion of using a reference measurement that may be precise, but not necessarily accurate, I believe there are therapy groups.
