about as many theories about seating depth as there are people seating bullets.
So .005" off the lands may be a good spot or not. You have to experiment to find
where your barrel wants to shoot.
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.
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.