Shoulder Bump Gauge Question

[Velocity]

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?

[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?

[Velocity]

Harpoon1

That does clear up the muddy water between the two gauges and the SAMMI spec.

Thanks a lot.

[Harpoon1]

Harpoon1

That does clear up the muddy water between the two gauges and the SAMMI spec.

Thanks a lot.

C069FF59-D08B-4C0B-9BF1-27085670C3DE.jpeg

[Dt219]

Harpoon is right on the money with his assessment

[LRRPF52]

[Velocity]

Harpoon1

I think I'll keep using the Whidden Gauge and put some oil on the Sinclair and plastic bag it.
Then again I may just put the Sinclair in the lathe and remove some of that deep champer from the face to match the Whidden Gauge.

[TR1Hemi]

The actual numbers are irrelevant. Measure your fired brass, adjust sizer using the same gauge, to achieve desired bump, usually .002-.003. The gauges are for getting a reference point. If you had a non adjustable bathroom scale that you knew read 5 lbs off, it does not matter over the long haul. 200lb one month v 210lb the next month guess what you gained 10 pounds regardless.

Straight from Sinclair "Rather than measuring to the datum line, these gauges indicate off of the shoulder of the case to get a better, more easily assessable measurement of how much you are actually sizing your cases."

One type of gauge uses a specific diameter of hole like Hornady, .330/.350/.375/.400, then Sinclair that just matches the shoulder angle regardless of diameter.

Oh and here is a HUGE BTW! When you measure your brass from your chamber make sure it was a mild load. Brass flow at the ejector hole or extractor cut or a cratered primer will TOTALLY jack your readings! Especially cratered primers, they will add 3-5 thou, you adjust for 4 thou bump, and you actually end up with zero bump. Because you are now measuring a primerless case.

[Kosh65]

I always measure after the decapping with a universal decapping die.

[TR1Hemi]

Perfectly acceptable, I also do not use the "anvil" that hornady sells with it's kit, not using it allows me to get caliper between the ejector and extractor flows.

[Velocity]

Perfectly acceptable, I also do not use the "anvil" that hornady sells with it's kit, not using it allows me to get caliper between the ejector and extractor flows.

TR1
Can you explain what you mean in the last part of your post "get caliper between the ejector and extractor flows".
And are you saying that the Sinclair Gauge will be more accurate because it uses the shoulder angle?

[BowChamp]

To me gauges are a reference, neither more accurate than the other. Sometimes one may be easier to use than the other. Just make sure you don't change a tool part during your sizing session and .002 will be .002.

I can see where cratered primers and such would give an inaccurate reading. This is what happens when something is changed.

[mtnlvr]

TR1
Can you explain what you mean in the last part of your post "get caliper between the ejector and extractor flows".
And are you saying that the Sinclair Gauge will be more accurate because it uses the shoulder angle?

I would say that the Sinclair gauge would hold its size better since it's indexing on a chamfer. That larger bearing surface will hold up longer than the edge of a hole. Not that it going to wear fast, but I suppose if you measure a lot of steel and are constantly rotating the case 5o get it to settle, we can occur. So with the Sinclair, it has a better chance of measure a headspace gauge over time without drifting.

[Velocity]

Thanks everyone, Sinclair it is.

[Harpoon1]

Thanks everyone, Sinclair it is.

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