Why different measurement when using anvil w/ micrometer

[MMH]

I know that you should use a anvil when measuring case length with a micrometer, but was surprised when I compared the length measured w/ & w/o. With the anvil, the 5.56 cases measured 0.004" shorter than with. This 0.004" was very consistent. I was careful not to put too much pressure on the case (especially w/o the anvil).

Can someone offer some insight as to why?

 

[dgeesaman]

Possibly high spots that don't contact when you don't use the anvil.

If you trim them and they don't have dings on either end, there should be no difference between the measurement methods.

 

[ronemus]

Any chance the cases were fired in a gas gun? That often produces bent rims, which make the case length dependent on how you align it with the calipers/micrometer.

 

[RegionRat]

Can someone offer some insight as to why?

Yes, many see the same problem between the wide area contact of the anvil base attachment, versus just the plain caliper jaws. Mine causes a 0.001" difference between with it and without it, but I had to fuss to make it that small or it would be larger. As a result, I rarely put that in the jaws.

One reason is that the anvil is almost never clamped onto the jaws orthogonal to the measurement axis. It is just the way that slot is milled and the way a set screw pushes it that make that very difficult to get square.

Once you have a wide area that isn't straight, you now add the area of the cartridge sample that also may have some profile errors, dings, primers sticking up, etc..

All of that adds up and means you pick one way or the other and stick with it, or make sure you record which one you are using and factor for it.

 

[Chiquita]

What kind of anvil are you using?

 

[RegionRat]

What kind of anvil are you using?

https://www.hornady.com/headspace-bushings#!/

Look down the page for

Anvil Base Kit​

Item #AB1

Notice there is no grind relief at the bottom of the cut. That means the sharp edge of the caliper jaw probably doesn't sit flat in that slot when the screw is tightened.

 

[Chiquita]

I found my Hornady base to have a bit of concave. I used a 2000 grit sand paper and and lapping compound to workout the concave.

 

[Chiquita]

Here you go.

Caliper bases from Dan

Shout out to Dan @delv7915 Shipped the two bases very quickly, in fact I received a tracking number before I processed the payment. Received them today (Saturday), put them on and it is very well made, perfect cuts, makes using the calipers easy to use. You are not fighting to get the base...

forum.accurateshooter.com

 

[Della]

^^ Also about a thou difference with Dans.