Am I correct in assuming that you had very little clearance in the neck of your 6BR bench rest rifle?

The amount of neck work hardening is relative to the amount of resizing required.

A reduction of 0.012″ per firing (like a factory chamber) will strain the brass much more than a tight neck chamber with brass that only needs to be reduced 0.002″-0.003″.

EDITOR: .003 Clearance with loaded brass in chamber.

As stated above, the brass hardens with each firing. As the brass hardens you get more spring back during neck resize and that eventually demands the next smaller bushing.

But what if you anneal the brass?

Well… Then you are back to square one and you should use the larger bushing again.

If after annealing you can achieve neck tension with a larger bushing than you initially used for new brass, interpret this as meaning your brass has been over annealed… or at least some of them were.

EDITOR: Yes changing bushings may be required. However, this need to switch may be over-emphasized. I shot a 6BR for many years with up to 8 firings between anneals with no need to change bushing and no loss of accuracy. Furthermore, after annealing, the originally-selected bushing still yielded the best accuracy. Going to one-size smaller bushing gave too much tension. While it involved a different system, I have had outstanding success with a honed FL-sizing die (no bushing), and again, there was no loss of accuracy even after repeated firings. That said, it’s certainly not a bad idea to have a bushing .001 less and .002 more than your current optimal bushing size — that gives you options. And if you find you have too much resistance to bullet seating (with your current bushing size), try leaving the carbon in the inside of the case neck after firings.

First of all you will never end up with only one bushing, more importantly you should have more than one bushing.

You will actually need progressively smaller bushings as the brass work hardens after firing…. Easily 0.002” smaller than you started with over the life of the brass.

Furthermore… sizing down from a fired round in a standard chamber to a size that will hold a bullet, creates a taper to the neck with the mouth smaller than toward the shoulder.

To remove this taper, resize the neck one more time with a bushing 0.001” smaller than the first.

You might even repeat one more time with a 0.001″ smaller bushing to get the neck walls really parallel.

You can feel a change in the resistance when cycling the lever on your press and can feel when its right.

You will also feel the difference when seating bullets.

Just make sure you finish at the size you need that creates your desired neck tension.