Rifle shooter wisdom gleaned from The Fouling Shot et. al. says: "...size the cast bullet to .0005 less than the diameter of the throat..." and, "...the bullet must fit the throat..."
After studying the diagrams of typical rifle chambers and reading many articles on the fitting of bullet to a particular rifle, I have observed the following.
All common (not custom) cast bullets have cylindrical driving bands. The "throat" of standard SAMII chambers are tapered.
Therefore, given a tapered throat (a 30-06 Ruger #1 in this instance), the actual measurement derived will be dependant upon where on the throat taper the measurement is taken. Up next to the chamber is .3135, further down towards the leade it is much less.
Just how then is a standard Saeco bullet with cylindrical driving bands fitted to a standard tapered throat by measuring? My practice up till now is to seat the bullet nose into the lands until it stops...I reason that the first driving band is then contacting the taper of the throat at some point along the taper. I have been sizing the bullets at .309, one thousanth over the .308 groove dia., been getting 100 yd. groups of slightly over an inch for 5 shots...most frequently 4 shots in under 3/4 inch, one shot usuall opending the group to 1 and 1/8 to 1 and 1/4 inch groups.
Regards,
dahermit