Shoulder Bump Gauge Question

[Harpoon1]

A few months back I purchased a Whidden Gunworks Shoulder Bump Gauge .350 which is aluminum.
Last week I purchased a Sinclair Shoulder Bump Gage 30A which is steel, I prefer steel over aluminum for gages just my preference.

My question is why do I get two different readings, I thought the target for a bump gage was the .350" diameter on the shoulder.
When I measure a new Lapua case with the Whidden Gunworks Shoulder Bump Gauge .350 I get a reading of 1.190" to base
and with the Sinclair Shoulder Bump Gage 30A I get 1.127". That's .063" difference between the gages and looking at the
SAMMI 6.5 Grendel drawing it shows the shoulder length of 1.220 -7.

What am I missing here?

I’d say it’s just the difference in the angle of the throat cut into each gauge, and where the shoulder is actually making contact with that particular gauge.

If you look at the Whidden bump gauge caliber list, many have different shoulder angles but use the same gauge. So, the gauge would actually contact different shoulder angles at different points. And I’m guessing not at the precise Saami spec measuring point either.

I believe the SAAMI spec measurement requires you have a gauge cut specifically for that calibers exact shoulder angle, and measured to a precise point on the shoulder.

The Whidden or Sinclair are both fine for measuring shoulder bump, you’re just looking for the relative change after resizing, not trying to match the Saami spec.