Since I originated this thread back in March maybe I should try to recap my side of it at least.
My original intent here was to see if there was a way to calculate the best range of seating depth adjustments for a load/gun combination to give the most accuracy you could squeeze out of it and with the least amount of testing. Reading the various books and manuals I have on reloading I saw that there was no hard and fast rule that everyone agreed on. I was wondering if there was an easy way to determine what the best increment for the adjustment is and what the best range of values for testing is. In the original thread there seemed to be a misconception that I thought I could calculate the “perfect load” without testing and this is not true.
NOTE: This seating depth adjustment was intended to be done after you’d found the best powder charge for the load. The Optimum Charge Weight method is the best way to do this that I’ve seen so far. You can find it at home.earthlink.net/~da...velopment/
At first I was looking at the “barrel whip” part of the barrel vibration, hence the thread title. I was thinking of this as a wave length/distance kind of problem. Then some of the good folks here pointed me at the Optimum Barrel Time paper online (http://www.the-long-family.com/OBT_paper.htm) and I realized that it was more of a timing problem. You want the bullet to exit the muzzle at a particular time to avoid the shock wave resulting from the powder charge going off. So I started down that path. Chris Long, the author of the OBT paper, was kind enough to answer my email with some good information. He can explain this far better than I can so I suggest you read his paper. I do know that the QuickLOAD internal ballistics software calculates the changes in pressure/velocity/barrel time when you enter a change in seating depth so I was hoping to find a quick and easy way to approximate it.
Some folks told me that I was ignoring all the other variables and I was, because I was trying to see if I could keep it as simple as possible. I didn't want to have to write a computer program to do it if a calculator would work.
Well, at this point I just can’t see an easy way to do it for two main reasons. As you change the seating depth you change the volume of the combustion chamber. First, if you change the seating depth on a .22 Hornet by 0.001” it results in about a 0.08% change in volume but if you make the same change in a .220 Swift it results in a 0.02% change. So you have to take into account the case volume. Second, as you change the combustion chamber volume you change the pressure and resulting velocity. Seating deeper increases pressure and velocity and seating farther out decreases them. However these changes aren’t linear over a very large change in seating depth.
You know, maybe if I can take the case volume into account and consider the pressure changes linear over a very short range of seating depth adjustments…