In short:

     1) Use feeler gauges to measure the distance from the end of the barrel to the breech face by closing the barrel on successively thicker gauges until one is snug

     2) Take the barrel OFF of the frame.

     3) Size your cases so they will protrude from the end of the barrel .001" less than the above barrel to frame dimension you obtained. You can compare the case head protrusion to your feeler gauges, or you can buy a machinist's depth mike. Careful measurements with a caliper are ok, too.

     4) Start a bullet into a properly sized case. Drop the case into the chamber, gently press the case head into the chamber, and note how far it sticks out of the barrel. The amount it sticks out is the amount the bullet must be seated deeper in the case until the head is back to the original position you obtained in 3) above.

     5) Turn the seat stem down a bit at a time and try the case in the chamber until the case head is back to its original position.

     Try it several times to verify you are at "zero." Once you find zero make a note of the overall cartridge length with that bullet. You can seat bullets more or less deeply from this point until you find where your barrel shoots its best.

     For fine tuning the seating depth, use the threads on the seating stem like a micrometer. Find what the number of threads per inch is on your seating stem. RCBS for example uses a 1/4x28 thread, or 28 threads per inch. Divide 1 inch by 28 to get the span between threads. In this case, it is .0357, which means that one full turn of the seat stem is about .036" up or down. Half a turn is .018," and 1/4 turn is .009," etc. 1/8 turn, half of a quarter turn, is .0045" and is under the .005" order of magnitude you need to work with. Half a quarter turn is easy enough to see.

     This is not a perfect world we live in, so you will often see that the bullet is not contacting the rifling perfectly straight on. Thus you will have some difficulty establishing exactly at what point the bullet is in contact with the rifling. Eliminating as much bullet run out in the case as possible makes this easier and also improves accuracy of course. All you can do is the best you can. I do everything I can to keep the chamber, throat, and bore all lined up, but seat dies often do not get bullets seated perfectly straight.