A subject of depth...pun intended.
I got a new rifle last week. Oh, not a new one, just somebody's old hand-me-down, and after a good clean up and examination, (It was really pretty clean) I set about to find the chamber dimensions and where the lands could be reached with several different bullets.
I record the serial number of the rifle and the weight and type of one particular bullet on my data sheet. One bullet per sheet which has a place for five different loads. This is usually all with one powder, but may be different. I used my Stoney Point seating depth gauge and modified case for the caliber (in this case, 338 WM) and push this particular bullet (a Nosler 225 grain partition) all the way into the chamber until it contacts the lands and stops. I then lock down the tool, extract the bullet and measure the length. I usually do this with a few bullets from the same box to get a good reading.
Then I record this dimension on the data sheet for this bullet under the title "Contact Length". This is the Cartridge Over All Length (COAL) with this bullet against the lands of this particular rifle. Then when I want to load and seat .020" off the lands I just seat it .020" shorter than the "Contact Length". I do these steps with all bullets of the caliber that I intend to use, or what is on my bench at the time. Each time I try a new bullet in this rifle, I measure again and start a new sheet. This keeps things straight and I don't loose my notes this way.
I develop data with each particular rifle and each brand of bullets (even of the same weight) with this technique because different bullets have different noses. With this systematic approach I'm able to determine with actual shooting test which seating depth I used and which the rifle prefers. Which of course is the real purpose. This can certainly be done without the Stoney Point system but it is much easier with it.
Now it must be noted that each individual bullet from the same box will have a slightly different nose length and this measurement will vary. The seating stem of the die however will contact the bullet much the same way the rifling does, below the nose at some point, so even though the COAL will vary by a few thousandths, the actual distance from the lands will be the same for each round loaded with the same die setting. There also is available an additional tool for the Stoney Point system to allow contact with the ogive of the bullet and may make this measurement easier for some. I have found that a few thousandths variation in length does not affect much of anything, accuracy or pressure, as long as you're not in full contact with the lands.
I then load ten rounds of each powder charge weight of the selected powder. In this case it was the 225 partition and RL-15 powder with FED 210M primers. On this first data sheet I loaded five different loads in one grain increments. I used RL-15 since this was a carbine length barrel and I was looking for low Standard deviations (SD) as indications of consistant powder burn. Now if your good in math you may have determined that I have loaded fifty rounds. I think that was last Thursday nite. Shooting took place on Friday and Saturday between rain showers.
These loads were fired and chronographed in two different rifles, five rounds each of each load, and each load of five was custom tailored to each rifle, at .020" off the lands. Rifle "A" the new carbine 19.3" barrel and rifle "B" an old proven 22.5" barreled rifle of the same make. There was a lot of cleaning in between the shooting. As it turned out, the carbine turned in high SD's and the longer rifle had low numbers so I think I'll try 215M (magnum primers) and do it all again. Maybe RL-15 needs more barrel or more spark, we'll see.
All these loads were in new Winchester brass which was neck sized with Imperial dry neck lube (graphite) then trimmed to .010" shorter than max length and deburred. I do not cut primer pockets to uniform depth or uniform the flash holes in brass until it is fired once. That will be done for the next firing. This fired brass is then measured at the webb to determine case expansion, and this measurement is also recorded for each load for each gun on the respective data sheet. I also have a place to record factory ammo case expansion and velocity, etc. but rarely buy factory ammo. This brass is also kept seperated (in small zip lock bags) until all measurements are taken and new primers are seated. This helps to determine if a load was excessive, seating new primers with little or no resistance means the pocket is stretched, a sign of excess pressure.
Now it seems I must do all this again this week end. This time just 25 rounds of the same charge schedule as before but with the hotter 215 primers. I don't know why I thought 210's would work. I won't test with the older rifle since I have that exact same data recorded already, with 215 and now 210 primers.
I'm sure glad I don't play golf! This is what I do, load, shoot, load, shoot, load.... Thanks for watchin'.