I should have said it's 'tricky' as there is no malice in it.
And this is where it get's tricky;
Concentricity is taken only to CENTER axis.
If I alter something .001" from concentric, it would measure .002" TIR at the same point.
However, CENTERED ammo as measured isn't always so chambered, because it isn't always straight.
Cartridge total indicated runout[TIR] combines factors; banana shaped cases, offset necks, thickness variance, seating misalignment, and relative measurement errors. When we subtract our minimum reading from the maximum and take the absolute value, the result is our Total Indicated Runout.
The best we can get is STRAIGHT ammo, and TIR quantifies this better than concentricity (given available methods of measurement).
Load a round with zero TIR(straight) and it is concentric.
Load a concentric round, and TIR could still be ugly(not straight).
The best tool I'm aware of to measure TIR(and I've tested a few including H&H), is Sinclair's: Concentricity Gauges - Sinclair Conc. Gauge w/ Digital Indicator
They refer to it as a concentricity gauge, but it's actually a runout fixture, and does not measure concentricity at all. That's ok though, it works. And I can tell you that your ammo(concentric or not) is NOT straight until measured so on this Sinclair(or variant of).
So what is the difference?
Picture your bent ammo as a jump rope, with both ends pinned as typical.
To best define it's maximum displacement from straight, you'd place an indicator in the center of it's arc and measure TIR. Right?
You wouldn't put the indicator nearest one pinned end, as it would show lower runout than maximum(actual). Unless, you don't care about runout, and just want any indication off concentric. Let's say concentricity that you can adjust with pressure on bullet's seated....
So what about runout? What about getting the ammo straight?
Now back to the jump rope. Only this time you pin the center of arc, and one end slid to the same plane, and indicate displacement nearest the free end.
Now your showing every bit of the devil in it, and this is what the Sinclair does.
Your ammo is NOT straight until it measures low enough in runout, with this method.
Is it then concentric as well?
Yes, of course.
Consider a bit of what bent ammo does when chambered(concentric or not)..
Will it be aligned with the center axis of your bore? NO, IT NEVER WILL BE.
How do you get your ammo to consistently point(wherever it does)?
You make it the same,, STRAIGHT