Caliper reading variability

[Labrat198]

I think I need a better caliper, but wanted to toss my scenario out to the group to confirm. I might also have too high of expectations, or any number of other things...

I know I am currently overworking my brass and trying to dial the shoulder bump back right now.

Using a 10mm case (I am weird, I don't have a 9mm yet) I get the following measurements:
Fired Brass 2.1625, 2.1605, 2.1595 Average: 2.1608
Resized Brass 2.1460, 2.1440, 2.1480 Average: 2.1460

Average shoulder bump is .0148

Research is telling me I should aim for .003 for an auto-loader to get the longest life out of my brass while still functioning reliably.

So here is the issue I am running into, the variability in my measurements is .004

My question is, is it normal for there to be that much variability when measuring? I get about the same amount of variability when measuring base to Ogive on my finished bullets. The Neck tension measurements are closer to .001

I am using Hornady dies with the quick change bushings in a Hornady Lock-n-Load single stage press. Caliper is a generic digital unit that I probably spent $20 on.

[ricsmall]

[A5BLASTER]

My advice is get the hornady tools for taking those measurements and throw that brass in the trash.

Use something built and designed for doing that job not some redneck'd tool set.

[ricsmall]

My advice is get the hornady tools for taking those measurements and throw that brass in the trash.

Use something built and designed for doing that job not some redneck'd tool set.

[Labrat198]

Well I'm not going to toss the brass in the trash, I reload those too.

I've seen a number of people, on this site too, recommend a 9mm case for a suitable alternative. I was hoping it would be a set it and forget it setting.

I've got the bullet comparator and using that to give a nice flat area that I can turn the brass back and forth to make sure I dont have any high or low spots.

It will be a few day before the "right" tool comes in the mail. Going to see what happens if I flip my case gauge over and use that.

Pretty sure the rest of my setup is solid, so the spring in the brass idea has me thinking. Wont hurt to check everything anyway though.

[Klem]

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.


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).

[grayfox]

With a setback of 0.015 (sorry but with a used pistol case I do not believe you have 4 significant digits in your measurements), vice the 0.003-0.004 you ought to be doing, you are over-working the brass bigtime. That conclusion one can reach.

But if you want precise specs/ measurements, you must use precise instruments, and a pistol case is not it. Might even want to look at your caliper, I would if I want to get that fine of a resolution. Even a slight but unnoticeable cant of that 10mm on your Grendel casing can cause some unknown few thousandths of a delta in your measure, one you won't know about and cannot eliminate or guarantee when it will showo an when it won't.

The other thing using the case comparator (with the 0.350 bushing) is you have numbers that others can relate to. And numbers you can relate to the Saami drawing. With the 10mm case you have neither of these.

[gwtx]

+1 on rotating the cases while applying pressure. I have found that there can sometimes be an almost invisible deformation of the surface caused by the extractor or ejector. You can feel it when rotating the case carefully, and can see the deviation in the reading on the calipers. Also, if your cases have the "dent" on the lip, that can cause variation in the reading, depending on how you hold the case, and whether or not you rotate it while measuring. On my equipment, a fired case measures 1.217 base to shoulder(350 bushing). I bump back to 1.215 (0.002), and have had no problems, and brass holds up well.

[Klem]

The other thing using the case comparator (with the 0.350 bushing) is you have numbers that others can relate to. And numbers you can relate to the Saami drawing. With the 10mm case you have neither of these.

Grey,

Be careful, even the Hornady gauge is not as precise as it could be. This makes comparisons with other gauges fraught. Not an issue if we are talking differences between two readings, or if using the same gauge of course and no doubt what the Stoney Pt engineers had in mind when they decided on how precise for mass production. If we want to compare against specs and other gauges we will need to machine our own.

My .350 is narrower.

[Labrat198]

So just to revisit now that I have the Hornady gauge and did a slightly larger set, I've got some numbers.

15 resized with multiply firings measured 3 times over 3 days:

SD= 0.001091
Spread= 0.0050

Largest Spread for a single case was 0.0010

5 Hornady Black 1 fired not resized measured 3 times over 3 days:

SD: 0.001077
Spread: 0.0035
Largest spread on a single case: .0010

control group 10 new out of the box hornady black

SD: 0.00041
Spread: 0.0010
Largest spread on a singe case 0.0005


So it looks like my calipers and methods are good. Learned something new about variability in the cases.

[grayfox]

Grey,

Be careful, even the Hornady gauge is not as precise as it could be. This makes comparisons with other gauges fraught. Not an issue if we are talking differences between two readings, or if using the same gauge of course and no doubt what the Stoney Pt engineers had in mind when they decided on how precise for mass production. If we want to compare against specs and other gauges we will need to machine our own.

My .350 is narrower.
[ATTACH=CONFIG]16198[/ATTACH]

True, certainly not trying to take it too far, this is a good point. Only thing I was meaning to add was that between the Hdy bushing readings and a 9 or 10mm case any reading will be totally useless for comparison purposes. At least you would be in the ball park with the Hdy tool.

[Chev2500]

Just another suggestion is to pop the primer out of the fired case before measuring. ( and the 10mm!)

[ported45]

And use the correct datum point diameter...the closer to the body gets bumped less than the datum point in the middle of the shoulder. The 10mm case, disregarding whether the mouth is truely square and is getting centered up on the shoulder to begin with, would be measuring too far toward the body. The "10mm datum point" could be getting worked 0.003", but the true datum point is getting set back farther and defeating the purpose you are striving toward.

It is not a HUGE amount more, but it matters. I was using the 400 bushing to begin with and then realized I should have been using the 350. Had to reset my sizing die after using the correct bushing.

And thanks to THIS SITE and its GREAT POSTERS for turning me onto the headspace tool because setting up my dies like I always had been show was giving me bump back like the OP and I was getting separating brass on the third firings!!

[FRB6.5]

Another point to consider with minimizing variability is possibly dedicating a second pair of calipers to the task. The gauge holder stays put on mine.

[Happy2Shoot]

Another point to consider with minimizing variability is possibly dedicating a second pair of calipers to the task. The gauge holder stays put on mine.

I have to admit, I did this too. I got tired of taking it off and then putting back on every other day.