Neck Bushing Size (RCBS instructions):
Proper neck bushing size can be determined two ways. Measure the outside neck diameter of your loaded cartridge, subtract .002" to .003". This allows approximately .001" of brass spring back for correct neck tension. The other is to measure the neck wall thickness of your case, multiply by two, add the diameter of your bullet and subtract .001". RCBS suggests that you have neck bushings one to two sizes above and below your target diameter.