My neck turning procedure (your mileage may vary):
#1 back off a full length or neck sizing die so it does NOT resize the case! Leave the expanding ball in without a decapping pin. This will expand the neck to a uniform diameter and ensure it's concentric.
MEASURE THE RESULTING INSIDE DIAMETER OF A SIZED CASE.
#2 Measure the mandrel diameter on your neck turning tool (this is the part that goes in the mouth of the case...)
COMPARE THIS MEASUREMENT TO STEP #1. The mandrel should be the exact same diameter of the sized case mouth, or no greater than .001 smaller than the inside diameter of the sized case mouth.
#3 measure the neck wall thickness of several sample sized cases. Take the outside diameter, less the inside diameter, divide by two. Compare this with a direct measurement to double check.
Unless you have a tight-neck chambered rifle, you'll want to remove a minimum of material. I normally choose to remove .002 from stock brass in a normal factory chamber. In a tight-necked chambered rifle, you will need to check with the barrel maker to find what you should turn to for a final outer diameter.
An example: I'm loading 308. My el-cheapo sacrificial RCBS 308 neck sizing die is backed off so that it ONLY functions as a neck expander. I expand some case necks. The average inside diameter is .305. The outside diameter is .330. My case wall thickness is .012 when measured directly (which matches the 0.0125 when mathematically calculated). My mandrel on my neck turner is .305. It's a tight fit to get the casings on it. When I turn the necks I pick a target thickness of .011 - removing 1.5 thou from the neck.
#4 Neck turn the casings.
#5 Size the casings with a bushing-capable neck sizing die. You MUST use a bushing die if you neck size because they don't touch the inside of the case neck! At this point you are pushing a case BACK to a dimension. Normal sizing dies expand on the retraction.
Why? Because at this point you've cut each cases wall thickness to a precise amount.
What size bushing do you get? You know the bullet diameter, and wall thickness, so subtract .002 and you got it.
Example: the afforementioned 308. I finish neck turning, and run the 308 through the Redding bushing die to finalize the neck. I pick a bushing diameter of .308+.011+.011-.002= 0.328.
This now gives me an absolute internal diameter of .306, and that precious "same exact amount of bullet tension in every case" that reduces velocity deviation.
PRIOR to seating bullets, it's a good idea to hand-chamfer the inside of the case neck (not mechanically!) to make sure they seat without "shaving" the sides of the bullet. I do this while I'm waiting for the RCBS electronic powder tosser to count out a charge.
#6 Seat your bullets, taking care to make the initial alignment as close as possible. Use a high grade seating die - Redding makes fine ones, so does Forster.
Done in this fashion, EVERY bullet you seat in a cartridge will have the exact same neck tension.
