How do you do it with seating depth first ?
There are different ways to do this. The fact that these methods can and do work to me is additional evidence to support a mechanism of seating depth optimization that is bullet-specific, rather than charge weight/pressure/velocity-specific. One example would be to select a slightly reduced charge weight (for safety reasons), and carry out a coarse increment seating depth test over a reasonably wide range, let's say in something like .005" seating depth increments. Once you find the seating depth region or window where a specific bullet appears to group the best, you can then pick a seating depth somewhere about in the middle of that region and start in with charge weight testing. I occasionally do this with a new-to-me bullet for which I have no idea where it might want to tune in with respect to seating depth. Once an optimal charge weight has been identified, one can then go back and cover a narrower range of seating depth in finer increments. Of course, there are many variations on this theme, but the general idea is that the initial seating depth optimum that is found will not change
dramatically, even if the charge weight/velocity/pressure are changed noticeably during the charge weight test phase. Even if it does change, one can usually find the new optimum during the fine increment testing without much trouble.
I have observed on numerous occasions that even when I found it necessary to change the charge weight markedly in the final load from that which was used during the preliminary coarse seating depth test, the seating depth optimum in the final fine increment testing didn't change much, if at all, from that identified in the initially, even though the velocity had changed significantly. I have observed this phenomena on many occasions with
jumped bullets. It may also occur with bullets seated into the rifling, but I almost never find it necessary to seat the bullets I use regularly into the rifling, so I don't. Of course, it is always possible that the same bullet, with two different charge weights, each having markedly different velocities, was still exiting the bore at the same point in the barrel harmonic cycle, and therefore exhibited the same seating depth optima. I think the odds of this happening so many time in loads with the same bullet but different velocities makes this a remote possibility. Not zero, but remote. That is one of the things about seating depth that makes me believe it is less about barrel harmonics than something else. Attempting to more precisely determine answers to such a question probably requires specialized equipment that I don't have.
The bottom line with optimal seating depth is that it is not difficult to determine empirically. We can simply do a seating depth test to determine what is optimal. Nonetheless, I would like to know exactly what we're actually doing when we optimize seating depth. I'm sure many reloaders fall into the same category. We
want to know exactly what is happening with every single thing we do. I believe having this information would be of benefit in the load development process. Unfortunately, I don't know the answer, so I do seating depth optimization as needed. Even though we want to know the answers, sometimes we have to settle for doing what works, even if we don't understand exactly
how it works.,
