I am wondering if anyone has found that neck turning their brass has accuracy improvement potential for our mostly straight-walled cases, lead-bullet guns? I know this is a useful procedure for modern rifles shooting bottle-neck cases and copper-jacketed bullets, but in our case, aren't we at least concerned with the bullet bumping up concentrically before entering the bore? If one has formed brass down from say, 45-70 to 40-65 size, I would expect the possibility that there would be some neck-area thickness non-uniformity occurring. Also, some chamber designs (40-65, at least) do try and have a cylindrical neck area, so maybe neck-turning for case-wall thickness uniformity might be doable here but not sure yet if it would be worth the effort. What might be the collective thinking on this?

I like to turn the necks till they average about half of the neck having brass removed.  I think it allows you to make ammunition with less bullet runout due to the more uniform neck thickness. 

Most full length dies will size the case more than needed even with less brass to size. If you neck size with buttons in the dies like the Redding dies you may need to get a sizing button the is a bit tighter.

And it is useless when breech seating.

Mes,
I would say that it is worse then useless for breechseating as it makes breechseaters nearly impossible to make and limits bullet choices to one particular  diameter.

I have not breech seated yet so no experience there.   

I have turned the inside of my "reamed" when using 405 brass in my 40-63.   I found that the small lip where the reamer bottomed out had more of a shoulder on one side than the other which indicates to me that control in that process is less than optimal and imperfections already present are left and possibly even exaggerated.   But in that case it is necessary as the chamber is too small to allow groove size bullets.   

Later I drew and purchased a reamer for a tight chamber in a 40-65 which I knew would require neck turning and got familiar with that process.  Again, this was necessary for a tight chamber.   

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After finding "turning" to be an easier process with more control on wall thickness I converted all my 40-63 brass to neck turned brass as well.    

I can't speak to accuracy improvements though since I started out with turned cases - meaning I didn't get a benchmark to measure group size against.   

I will say it gives you some control over fit in the chamber.   I have an old Marlin 1895 with a chamber neck that is only .250 long.  If I want to minimize the brass sizing I can turn about .300 on those cases and have very little change with an optimal fit in the chamber.   On my Ballard 40-65 I can lengthen how deep I chose to turn and again have a near mirror fit to the chamber.   That and a bushing die allows me to do less sizing.

Breech-seating was your great grandfather's work-around for neck turning.

40 Rod