Jimm,
It does seem that just about any load recipe will have various seating depths which will allow the groups to form well.
Often, benchrest shooters really don't know the "whys" behind what they do. Like a top level Nascar driver, he's great at what he does, but he really doesn't know
why this particular cam works better on this particular track. He only knows that it does. This is why we often get a mish-mash of blather from BR circles with regard to seating close to the lands.
When I hear that a particular bullet "likes" a certain amount of jump to the lands, I am skeptical. In nearly every situation, you can make that bullet shoot well at magazine length. Folks who believe that bullet X needs a .020" jump to the lands will begin with that seating depth and adjust the powder charge until they get tiny groups. They conclude that indeed this bullet does "like" a .020" jump.
Barnes and perhaps some other makers have found that some of their bullets do better when loaded close to the lands. Again, they tell you this without telling you why. /ubbthreads/images/graemlins/laugh.gif Likely, the subject bullets are difficult to seat straight, and they end up with higher than average runout (perhaps the ogives are not compatible with typical seater die buttons). The closer seating to the lands may allow the bullet to be deflected to center as it engages the lands, rather than smashing into the lands and engraving off axis--as might happen with a deeper seat.
I think if you seat the bullet straight, and your case neck and chamber are concentric, you can seat deeper and get great accuracy.
Changing the seating depth changes the
barrel time. Bullets seated at different depths will be released on various points on the vibration pattern at the muzzle.
As you can see from this diagram, there are likely two different accuracy nodes on a barrel's vibration whip. Altering the seating depth will move the bullet's release to different points on this (typical) figure 8 pattern. As you seat deeper and deeper, or shallower and shallower, you'll move the bullet around on this pattern. Ideally, you will want the bullet to release as close to the narrow loop's endpoint as possible. Here the muzzle is slowing, almost to a stop, before it changes direction. Bullets with muzzle velocities within about 25 fps of ES can still be released in virtually the same point in space--a good thing.
If you're releasing your bullets on a "straightaway" in the figure 8, they may string on the target--even if the ES looks great on the chronograph. This is why it is always best to let the target over-rule the chronograph in situations where they don't agree. /ubbthreads/images/graemlins/wink.gif
The vibration whip is not to be confused with the main barrel shock wave, as identified by engineer Chris Long. Chris Long's pages can be linked by going to my website and scrolling to the bottom of page one. That's a different thing entirely, and OCW load development should be used to identify the "coarse tune" powder charge, and then seating depth adjustments should be used to make the final tweaks to the load. Most guys--even vaunted BR guys--get this backwards. They begin with a fixed seating depth (yeah, because so and so said this bullet "likes" this seating depth /ubbthreads/images/graemlins/smirk.gif ) and they manipulate the powder charge to make that seating depth work. This is much like beginning with a particular ignition time setting on a racing engine, and then changing pistons until you get the timing to work. /ubbthreads/images/graemlins/tongue.gif /ubbthreads/images/graemlins/laugh.gif
So yes, there are indeed different points of seating depth that will work just fine--for the reasons mentioned above...
Dan
