Note that only their 6mm turner is .004 below nominal bullet (at .2390). The other mandrels (22, 6.5, 30 cal) all mic at .003 below nominal bullet diameter, so in theory (Hornady's theory, anyway) if you use their New Dimension die w/ expander plug, and there is .001 spring-back, the neck IDs should be left .001 larger than the turner mandrel. Or, you could use a Sinclair 6mm expander mandrel which is .2422 but that will be too large unless you get .001 spring-back from it (not likely from a mandrel, vs the Hornady pull-through plug).
So still sorting out the expander aspect, to be continued ...
Update:
Sinclair sells (separately) pairs of mandrels, a turning mandrel and an expander mandrel, which they state are ideally sized for prepping and turning. Assuming that's true, I decided to find out exactly how much larger their 6mm expander mandrel is than the associated 6mm turner mandrel.
Note that Sinclair state:
Our neck turning tool mandrels measure approximately about .002" less than the bullet diameter for the caliber. Our expander mandrels are made .001" larger than the turning mandrels. Using an expander mandrel prior to neck turning sizes the case to fit the neck turning mandrels perfectly.
The ".001 larger" turns out not to be true, because I ordered both 6mm mandrels from Sinclair (turner and expander) to check them out. It turns out they are sized as follows (measured using two different .0001" micrometers):
6mm expanding: .2422
6mm turning: .2409
Difference: .0013
So in Sinclair's world, a .0013: difference between expander and turner is ideal. Assuming something like .0010" neck spring-back from the expander, that would leave ~.0003" clearance between the case neck and turning mandrel, which seems about right.
Note: The two types of Sinclair mandrels are identical - same form factor and shape - so a "turning" mandrel can be used as an "expander" mandrel, in the Sinclair expander die.
Recall the Hornady turner mandrel I received measures .2390". Now if I used a Sinclair 6mm
turner mandrel as an expander, the relationship becomes:
Sinclair turning: .2409
Hornady turner: .2390
Difference: .0019
This is probably acceptable but it's still potentially .0006" more slop between case neck and turning mandrel than Sinclair's ideal.
I called Hornady and they told me my .2390" turning mandrel is at the very
bottom of their spec of .2390 - .2395 for the 6mm mandrel. A chap there offered to take a mic and poke through their bin of mandrels to find a larger one, which he did - a .2394". Hornady sent me that "oversize" mandrel free of charge.
So with the Hornady "oversize" turner mandrel, and the Sinclair
turner mandrel used as the expander, I get:
Sinclair turning: .2409 (as an expander)
Hornady turner: .2394
Difference: .0015
Bingo! I'm now within .0002" of what Sinclair considers ideal. Long-winded explanation, but there it is.