Hornady headspace comparator?

[birdiemc]

Let's see if I got this right. Hornady says 308 win headspace comparator is .400". SAMMI spec shows datum for 308 win is .400". So it seems they are trying to measure to datum...if tolerances are perfect at Hornady, And in the chamber, and on the brass then it works....so in the real world it would seem the fact that it is a "comparator" is the only thing that provides value to the tool?
So apply that to measuring bump for shoulders....many say use hornady headspace comparator. I am cheap. I have the bullet comparator set, there's bushings in there big enough to fit over the shoulder on my cases so I use them. So what if they arent .400 (odds are neither is the headspace comparator, precisely) flashback to geometry tells me that as long as the angle is the same from case to case, the measurement will work to compare the two, right? And if the angle varied, the bullet comparator would provide no more error than the headspace comparator right?
Or have I just got this all wrong?

 

[MagnumManiac]

The comparator collets are not precisely .375", .400" or .420", but they ARE consistent.
This is all that's important.
As long as you can measure to the same point on the case, then you can measure how much brass you're moving when sizing.
I use mine for imperfect measurements, then for precise measurement I use a RCBS Precision Mic that has no chamfer where it touches the case like the Hornady tool does.

Cheers.