Thank you for this reply.
When loading do you set your resize die to the -0.002 or do you go past that to like -.003 or 4? I don't crimp so i'm just concerned about how to set neck tension if the die isn't touching the case after firing.
This is a long post, sorry about that. You asked about neck tension.
The function of full-length sizing is to reduce body diameter, push the shoulder back, and reduce the neck diameter. All of these changes are small values. In your case, the shoulder did not get longer - you measured the length with your gauge and you see that it is the same after firing as it was before firing. However, the case body and the neck both expand in a radial direction. In my case, I am shooting a 308. My chambers have a .340 neck inside diameter and after firing my brass measures .340 outside diameter. When I lube the case and force it into the die, my neck OD is reduced to about .330 - roughly, I don't recall exactly but .330 is close enough. With a .330 OD, the next ID is about .300 - more or less. This happens without respect to the shoulder length. Same for the body diameter, it is reduced regardless of what happens to the shoulder. For what it's worth, by the time the brass shoulder is pushed far enough into the die to run into the die shoulder, the neck is completely sized.
After running my brass into my full length die and reducing the neck OD, I screw an expander body with mandrel into my press and run every piece of brass over that mandrel. The mandrel diameter is about .305. Brass is slightly elastic so the mandrel expands the neck ID from about .300 to about .305 or .306 with some elastic tension. When I pull the case off the mandrel, the elastic tension relaxes and the neck ID becomes .304. I check a few cases with a .304 pin gauge just to be sure.
Firing and sizing causes brass to migrate from the middle of the body toward base and toward the shoulder and neck. This moving brass forms a donut at the base of the neck. In the olden days they were called "dreaded donuts". There is probably a new name now. When you size the case, the OD of the neck is forced to be a cylinder or nearly-cylindrical-cone with the inside of the neck thin at the top and fatter at the donut. Forcing the neck over the mandrel has the effect of making the neck ID cylindrical and the neck OD irregular. That is fine and actually helps with bullet seating and centering the round in the chamber prior to firing.
I have a 308 with a SAAMI minimum length chamber and another one that is .002 longer. With my RCBS gauge, the short chamber measures -.002 and the long chamber measures 0.000. Insert my headspace gauge (the GO gauge) into the shorter chamber and the bolt almost by not quite closes when the barrel is torqued to 30 ft-lbs. With the longer chamber, the bolt closes when the gauge is in the chamber. Brass fired in the shorter chamber comes out about chamber length or just a smidge longer - about 0.000 using the RCBS gauge. Again, brass is elastic. I set the die so that when I lower the press handle, the shoulder length is about minus .002 from the chamber length. When I remove the case from the die, the elastic tension relaxes and the case comes out chamber length plus or minus .001. Brass is imperfect so individual pieces get slightly different results.
Since I have two guns with different chambers, sometimes I make different ammo for chamber A and B and sometimes I make ammo that is between the lengths. On the gun with the long chamber, the bolt closes freely. On the gun with the shorter chamber, the bolt is a bit firm.
Right now I have about 150 pieces of brass ready for charging. There is an auto-throw / auto-trickle device in my future. Friday night I rearranged my scopes. Saturday, I shot for 100-yard zero and slipped turrets. Today I fired 90 rounds, about 55 rounds in the long-chambered gun and the rest in the other gun. After shooting today I now have good gun data for both guns on steel every 100 yards out to 1k.
