LabR,
Thanks to caliper error, brass quality and variations in springback you will never get exactly the same measurements across cases; it's more about averages. That said, it pays to keep the standard deviation of your batch low (meaning less difference across a range), by buying quality brass and having good gear and being careful.
Here are the stats for 10 fired cases using digital calipers (Mitutoyo) with the Hornady gauge, and the same batch measured with a new/unfired 9mm case.
[ATTACH=CONFIG]16187[/ATTACH]
Lapua cases, several times fired, unsized.
Firstly, I suggest you measure more than three to get more meaningful stats. The chances of your metrics being unrepresentative of the whole batch increases the smaller the sample size.
It looks like when measuring with the Hornady gauge there is less variation. With the only variable changed being the gauge it says something about how the Hornady gives more exacting results. No surprises here.
The variation across 10 cases base-ogive is much lower than yours. This could be from several reasons. Better calipers, better gauge, better brass and perhaps even the way we measure (I rotate the case gently to settle the bind). I also note it takes longer and is more fiddly to settle the cases when using the 9mm as a gauge. Again, no surprises here.
You are doing this to achieve an outcome; setting the die at the minimum bump that will guarantee reliable cycling. Another way of doing all this is undersize and shoot and then gradually screw in the die until cycling becomes reliable. And not measure anything. .003" is just a benchmark and it may be more or less in your gun.
(Note: In the graph, the numbers on the same line in the chart are not the same case).
Thanks to caliper error, brass quality and variations in springback you will never get exactly the same measurements across cases; it's more about averages. That said, it pays to keep the standard deviation of your batch low (meaning less difference across a range), by buying quality brass and having good gear and being careful.
Here are the stats for 10 fired cases using digital calipers (Mitutoyo) with the Hornady gauge, and the same batch measured with a new/unfired 9mm case.
[ATTACH=CONFIG]16187[/ATTACH]
Lapua cases, several times fired, unsized.
Firstly, I suggest you measure more than three to get more meaningful stats. The chances of your metrics being unrepresentative of the whole batch increases the smaller the sample size.
It looks like when measuring with the Hornady gauge there is less variation. With the only variable changed being the gauge it says something about how the Hornady gives more exacting results. No surprises here.
The variation across 10 cases base-ogive is much lower than yours. This could be from several reasons. Better calipers, better gauge, better brass and perhaps even the way we measure (I rotate the case gently to settle the bind). I also note it takes longer and is more fiddly to settle the cases when using the 9mm as a gauge. Again, no surprises here.
You are doing this to achieve an outcome; setting the die at the minimum bump that will guarantee reliable cycling. Another way of doing all this is undersize and shoot and then gradually screw in the die until cycling becomes reliable. And not measure anything. .003" is just a benchmark and it may be more or less in your gun.
(Note: In the graph, the numbers on the same line in the chart are not the same case).