What is considered an acceptable level of case neck runout? I know we would all like to see zeros however, when that is not achievable what is considered acceptable. I remove the decapping pin and expander ball from the FL length die when I resize. I can have 0-.001 runout from the FL sizing die. However, when I expand the neck with a mandrel die and a gauge pin that is 0.002 less than the bullet diameter I get .003-.005" runout on the necks. I have tried allowing the mandrel die to float in the press so it will self align, however, don't see any improvement in the results.

0.0"~ .002" goes into the competition ammo box. .003"~ .005" gets straightened to <.002" and is reserved for fouling shots. Anything >.005" (if it happens) is reserved for dicking around on plates. All values are TIR.

How uniform are the case necks?

This ^^.
Easy to have .002"-.004" variance in neck thickness. The full length die uniforms the o/s of the case, pushing any variance to the inside. The mandrel die pushes the variance back to the outside.

After thinking about what I am seeing this is the next thing to check. With Laupa brass I see .002 after mandrel neck sizing. With RP I am seeing .005. I am thinking neck turning to get uniform neck thickness is in order.

The problem with neck-turning brass that has .005 inch variation in neck thickness is that variation usually extends into the case body as well--which results in cases that are often slightly curved overall themselves. This makes it more difficult to minimize seated bullet run-out.

All of which is why it's far better to start with more uniform brass in the first place.

^^^^^
This
Check and turn your necks if needed.

What's your game?

Hunting - What runout? Anything goes.
Highpower? I don't check, but around .004" or less and I'm happy. Multi-lug actions like in the AR aren't as sensitive to runout as 2 lug bolts.
Benchrest? You want to win? How badly?

for low run out and consistent neck tension i do the following:
A. full length body size and bump shoulder 0.002-0.004 depending on what your chamber needs.
B. bushing size neck to enable 0.0005-0.0010 I/D cleanup. This will remove donut brass flow
C. Neck turn using oversize pilot hard turned to provide tight fit to reamed neck I/D.
D. Bushing size for 0.003" press fit to bullet

Fellers, I fireform first with a stiff load of pistol powder and a wax plug instead of a bullet. This way you start with a straight case. I used to be in the carbide neck bushing business and neck size only to a slip fit for the mandrel of my Pumpkin neck turner. On my new brass I don't want any expander or sizing die touching it until after I fireform, neck size, and turn.

What is the game those cases are used for, Butch?

This is for my 30BR and 6ppc cases. I am not a range hound and on my hunting rifles I use a combo that shoots under 1"MOA with a cold, hot, dirty, or clean barrel. My personal hunting limit is 300 yards. As I don't wear out components at the range I buy the best brass that is available. Basically with my hunting brass I fireform and trim to length.

Corollary: how many times in a row must a measurement repeat before it is accepted as the true measure?

There is no “true measure”, only a true dimension which can never be measured with arbitrary precision. The uncertainty about the measurement agreeing with the true dimension decreases with each additional measurement. It’s a judgement call determining how many measurements are “good enough”, depending on your uncertainty requirements.

Fantastic!

LOL

I'm most concerned with a fouled and cold barrel. I do not go hunting with a clean barrel. Probably doesn't matter, but that is my practice.