ekp,
The exercises that you are being lead through are exactly the ones I was in the process of discovering about 7-8 years ago... that was how I found competitive short-range group shooting, and I've been hooked every since. Many barrels and thousands of rounds later, I'm still learning little tweaks to get the bullets to hang together.
Everyone has a strategy, here's mine...
I want to know a couple of numbers... first, what is the maximum seating depth (jam) before the rifling starts doing it's own seating. Second, what is the minimum depth when the bullet is no longer touching the lands at all. Be careful when you hear someone describing the seating depth of their their pet load, since some people talk "jam-minus", and some talk "touch-plus". Additionally, the measurements can be quite different for any bullets produced on different dies or even different lot numbers. They can also change with barrel wear, so it is something I check quite frequently (almost every match throughout the shooting season).
First, for a particular bullet, I like to know the max length that it can be seated out... just seat a bullet a little long in a FL resized case (no powder or primer please) with your normal neck tension, close the bolt on it (I do that step twice), then extract it. If your tension is too heavy (or light) and the bullet sticks, then start with the bullet closer to the final seating depth (within say .005) It should move back enough to measure, but not enough to stick. I will usually leave this cartridge in the box of bullets as a reference that I can go back to. I loosen the set screw on my micrometer-topped seater and gently spin it down on the reference cartridge. The seater top will spin with the micrometer top until the stem mouth contacts the bullet. This becomes my the "jam" length.
The second number takes a little more "feel". Remove the firing pin assembly from the bolt (required -- you can't feel the lands with the cocking spring in the system). Prepare a second case and bullet at max jam length and start increasing the seating depth a couple thousands at a time, chamber the round and "feeling" the bullet twist against the rifling lands. On my competition gun, usually somewhere between .025 and .035 off the jam length, the feel of the bolt drop has a definite change and I know that I have found the length where the bullet is no longer in the rifling. Seated any deeper and the bullet will be "jumping" to the lands. Other methods for finding this involve polishing the bullet and looking for rifling marks, but I learned this one from Jack Neary and I think the bolt feel is more reliable and repeatable.
When I'm working up a load for a new barrel, testing a lot of bullets, or practicing before a match, I like to go through a sequence of light-to-heavy powder charges (usually about .3 gr apart) at the jam length with 3-shot groups, looking for nodes that show some promise. Then picking the best-grouping charges, I try to "fine-tune" it by trying testing charges at, just above (.1gr), and just below (.1gr) with seating depth increased by .005, .010, .015, and sometimes .020". The holy grail is a charge/depth combination that does not "blow up" with the minor variations. The errors of all the components of our systems accumulate, and the real gem is a load that is both accurate and error-tolerant.
The key is to find what the gun likes and not be too hung up on making the gun shoot what you like. Grandpa used to tell me I could catch more fish with worms than with cheesecake, no matter how much I liked cheesecake better.
Rod
