Thanks Mike, I appreciate your thoughtful reply. Never hurts to ask, right?Bullet seating is no more tuning than primer swapping is.
Both are prerequisite to best tune,, they can both take you out of tune, but you cannot actually tune with either.
You tune with powder(right to the kernel) and neck tension.
Full seating testing, as Berger recommends, is a coarse process to find the land relationship that a bullet likes in YOUR barrel. This cannot be predicted but has to be tested for. JUST LIKE PRIMER SWAPPING.
Both cause huge dispersion changes, way beyond that of actual tuning with powder.
With full seating testing, you can make a 1/4moa gun open to 3/4moa & back. No amount of powder change(type or amount) can do that, and you will not recover from worst seating or worst primers -with tuning.
That's why it's vital, right up front, to determine best bullet seating, and best primer, as prerequisites to tuning.
Also, once you have determined best seating with a bullet/barrel, and best primer for powder/cartridge, this does not change. Even if you start load development over with a different powder, best seating remains just the same.
It is independent of tune.
If ~50yrs ago they knew just this much about it, then by now we would have our chambers cut to standards for particular bullets, and we could look up on a table exactly where best seating is.
You know what progress we've made in this regard, for ~100yrs? NONE
So your endeavor to understand this is great, but you won't learn anything by asking around. It's just the way it is with small arms and humans in general.
There are several articles on how to do it...but none answering the questions I am asking...
The link above has some good information and test results done on seating depth, maybe this will help.
I got a LOT of information from them on bullet seating and lot more. Not to step on toes, but experts such as Erik Cortina seem to indicate that getting the seating depth for the best accuracy is part of the tuning process for the best accuracy along with powder variations, primer swapping, neck tension, and whatever.That's a good idea, Berger might be a prime candidate since they actually have a recommended procedure. I will think about that
I am not questioning it, I am trying to understand the what/whyI got a LOT of information from them on bullet seating and lot more. Not to step on toes, but experts such as Erik Cortina seem to indicate that getting the seating depth for the best accuracy is part of the tuning process for the best accuracy along with powder variations, primer swapping, neck tension, and whatever.
I am not looking for anyone to teach me how to figure out what the what the "bullet jump" should be in my rifle. You are right, its not rocket science, just read the question and if you can not answer it, don't!It's not rocket science so I'll keep it simple. All chambers vary re lead into the lands, slow powders usually shoot better well off the lands . Fast powders are usually better on or just off, however there are exceptions so start about 10-25 thou off then try by increasing the jump 10 thou at a time. I wouldn't think going more than 60 thou is necessary. You will be able to tell from the horizontal or vertical group which way to go in or out. One rifle I had used to shoot 1/4' 15 though off and after rebarreling it needed to be 55 thou off the lands to produce that sort of accuracy.
JeffWith the knowledge of this group, I think the reason that you haven't gotten an answer to your question is because I don't think it really can be answered by an equation. The variables that would need to be measured and inputted into an equation would be almost impossible. For example, two identical barrels (what we would call identical) are built and chambered. The steel will have its own imperfections at different locations, the rifling process is impossible to perfectly duplicate, the chamber reamer will have different amounts of wear and lathe setup will all have an effect on how a barrel responds harmonically and how seating depth will effect accuracy. It was suggested before to speak to the bullet manufacturers and I believe that would be a great resource to understand a little more about why certain bullet designs are more forgiving in terms of seating depth, but all other factors (mostly human and tool wear) nearly prohibit a mathematical solution to what seating depth is the best and why it is.
Dragoon,I think the answer to your question is simpler than we want it to be. I am a machine design engineer by trade, and live in the precision machining world. I also enjoy shooting, and even more tinkering with my guns. In my experience with my primary precision rifle, a Desert Tech SRS 338 Lapua, I wanted to utilize all of the case capacity while using Berger 300 grain OTM bullets. I wanted to seat the bullet so the boat tail shank junction was at the case neck shoulder junction giving me the most capacity and the full neck for retention and bearing surface. Then I throated the chamber to give me .010" off the rifling so as to not jam the bullet into the rifling because I need to be able to unload without leaving the bullet in the chamber, simple as that. The OTM Hybrids are supposed to be "more tolerant" to jump and I must say that is true in my case, I did not see any real difference initially in testing, and now after 800 plus rounds with the throat eroded .020" more the accuracy is still 1/4 MOA. Mechanically the design shape of the bullet and the lead into the rifling are what make jump more or less tolerable. The benchrest guy's jam their bullets into the rifling because it is more accurate, as the bullet is started into the rifling before ignition. Practical shooters can't do this because they could induce a problem no one wants during unloading, so they must stay back some amount. Manufacturers design more throat into a rifle cartridge to allow for a greater range of bullets, but I believe mostly to reduce pressure as in Weatherby magnums and Remington ultra magnums where they have .300" to .400" to be safe. If you take a .300 ultra mag and load it with a 110 grain bullet it will space walk before entering the rifling. This is the other end of the spectrum from the benchrest guy's! So, I can only say what I know to be true in my case, I had lots of bearing contact in the case neck and only .010" to the rifling (when new) and this helps. I just had Dave Manson make a reamer to my specifications for my Lapua, and one for a 6.5 PRC with Berger 156 grain EOL's and plan on doing the same. I will see how it works out, but am quite sure it will with .010" jump and tuning my load to suit. Hope I didn't get too long winded and this helps.