300 WSM Headspace question

[lobo55]

I have a Kimber 300 WSM that I have been reloading for. I have a hornady headspace guage and the RCBS precision mic. The measurements are:
Hornady headspace guage measurements:
Comparator + 0.420" bushing + Fireformed 300 WSM case= 3.741"
Comparator + 0.420" bushing = 2.003"
Headspace = 1.738"
RCBS Precision Mic measurements:
1. Fireformed 300 WSM case= 1.7260"
2. Fireformed 300 WSM case= 1.7260"
3. Fireformed 300 WSM case= 1.7258"
4. Fireformed 300 WSM case= 1.7258"
5. Fireformed 300 WSM case= 1.7260"

Average headspace using RCBS precision mic= 1.7259"

RCBS precision mic table shows minimum dimension-1.7260" maximum dimension-1.7360"
so there is a 0.012" difference using 2 different methods. I know as long as i stay consistent with measuring and setting my resizing die there will be no problem but my question is-there any reason anyone can think of why there is such a big difference?

Thanks-lobo55

[Popgunner]

The RCBS precicion mic is made just for your 300wsm & has the shoulder angle just the same as your cartridges & I would guess it would have a more perfect datum spot. I would trust the precision mic to give your true headspace more than the Hornady.
The Hornady is a good unit & can be trusted to give good "relative" results. Hornady's 5 die set is advertised to cover 17 calibers. Problem is the shoulder angles can be different & the datum point changes. I wouldn't try for an exact reading with the Hornady, you just use it in a relative way to adjust your sizing die to set the shoulder where you want it. Works great for that.

[lobo55]

Thanks Popgunner.....