If I can't trust my caliper, what can I trust?!?

1
I started my foray into reloading this week... starting off with 9mm. That part is going pretty well. Worked up some loads with a few different powders... it's great.

It took about 1 hour of reloading to realize... "Shit man! I need more bullets!" About 5 minutes after that I had the thought... "I really need a King Hell amount of bullets!" This made me consider cast bullets even more so and the first thing to do, it seems, when traveling down that road is to slug one's barrel. I did that... two times.

In measuring the slugs I'm coming up with numbers that just don't appear jive. Grooves at .351-ish and lands at .345ish. These seem quite a bit off from what appears to be standard, so I pulled out a Rainier plated bullet that the company says is .355 and my $25 Lyman SS dial caliper (same used for all these measurements) is telling me the diameter is .352-.353.

Now the question - and my suspicion is that the answer is "yes".

Can my calipers be off as much as .005?

I'd love to have calipers that reliably show .0001" but that seems like a pipe dream if I'm unwilling to drop several hundred in loot. Anyone have a recommendation on reliable calipers in the "around or under hundred dollar" range? Am I expecting too much to have a pair at that price that is only ±.001"?

Any help appreciated.

Oh... and have a terrific Saturday!
“We cannot be sure of having something to live for unless we are willing to die for it.”
― Ernesto Che Guevara

Re: If I can't trust my caliper, what can I trust?!?

2
On every cast bullet specific site that I have ever been on the general consensus is that no caliper can be trusted with measuring bore and chamber measurments. Those tasks require micrometers and pin gauges. If a person really throws themselves into this hobby expecting to find perfection they can expect to pay a fortune for very expensive instruments. I've gotten by with out those intruments mainly because I don't compete in a precision sport like cast bullet benchrest shooting or IHMSA competition that includes a bank of targets at 200 meters. The only instance of leading that I have experienced is with my BFR in 45colt when I have shot plain based bullets at the extreme velocity and chamber pressure the gun is capable of. They where very unpleasant loads, if I ever decide to punish myself regulary in the future I will buy a new mold that casts bullets that call for a gas check. Non gas checked bullets will skid a bit that results in leading and mediocre accuracy at long range. Don't over think it because trial and error in the senible range will tell you what you need, with experience.

Re: If I can't trust my caliper, what can I trust?!?

4
I disagree with Pop. Mitutoyo makes high quality digital calipers that cert to .001 and resolve to 4 places, and can be had for around a bill. There are a lot of variants. Hit me up if you want some help. I gave up on the junkers.
As far measuring bores and throats, gauge pins are probably the best way to get meaningful measurements. That need not be an expensive endeavor since you can pick just the gauges you think you need instead of buying the whole set. But I would agree with eelj that pin gauges are not at all necessary. A good mike, however, is immensely useful, so long as you recognize its limitations.
Hell is where:
The British are the chefs
The Swiss are the lovers
The French are the mechanics
The Italians make everything run on time
And the Germans are the police

Re: If I can't trust my caliper, what can I trust?!?

6
Calipers have a problem called Abbe (ab-bay) error. Happens with expensive ones too, although I agree with Beaurrr more expensive calipers measure better. I use Mitutoyos and Starretts, both of them good to .001". Other brand names are Mahr, Brown and Sharpe, Eatalon and Tesa.

Undersize on a caliper is pretty common newb mistake, you are rolling the thumbwheel too tight. Abbe error is the error caused by the scale being offset to the the measuring faces, which is true of all calipers. What happens is the scale is reading low because you are actually bending the frame + jaws of the caliper with too high a force and the Abbe error is the resulting distortion in the reading. Your force should be low, just enough force to keep the caliper in contact with the work, no more. Comparing it to trigger force, it should be about the force of light SA trigger, no more than 2 lbs which isn't much.

I should also mention to clean the jaws by pulling a clean paper through them, and check to see that at the start of you meaurement when the jaws are closed you have zero.

One thing you might try is measuring pennies or nickels. Pennies are .750" and nickels are .835", the tolerances for production are I think ­±0.001", so measure like 3-5 of them. Turn them as you measure them, they are pretty round and you should get very close to the same reading as you turn. make sure you aren't tipping them in relation to the jaws as you do this. Your average should be close to the nominal size I listed. If it's still under your pressure is still high. If it's over then I suspect the coin isn't aligned very well.
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Re: If I can't trust my caliper, what can I trust?!?

7
Antiquus wrote:Calipers have a problem called Abbe (ab-bay) error. Happens with expensive ones too, although I agree with Beaurrr more expensive calipers measure better. I use Mitutoyos anSupremely helpful, thanksd Starretts, both of them good to .001". Other brand names are Mahr, Brown and Sharpe, Eatalon and Tesa.

Undersize on a caliper is pretty common newb mistake, you are rolling the thumbwheel too tight. Abbe error is the error caused by the scale being offset to the the measuring faces, which is true of all calipers. What happens is the scale is reading low because you are actually bending the frame + jaws of the caliper with too high a force and the Abbe error is the resulting distortion in the reading. Your force should be low, just enough force to keep the caliper in contact with the work, no more. Comparing it to trigger force, it should be about the force of light SA trigger, no more than 2 lbs which isn't much.

I should also mention to clean the jaws by pulling a clean paper through them, and check to see that at the start of you meaurement when the jaws are closed you have zero.

One thing you might try is measuring pennies or nickels. Pennies are .750" and nickels are .835", the tolerances for production are I think ­±0.001", so measure like 3-5 of them. Turn them as you measure them, they are pretty round and you should get very close to the same reading as you turn. make sure you aren't tipping them in relation to the jaws as you do this. Your average should be close to the nominal size I listed. If it's still under your pressure is still high. If it's over then I suspect the coin isn't aligned very well.
Supremely helpful - thanks mucho! After adjusting for my ignorance, the slugs are telling me I have a .355 barrel... I can certainly go from there.
“We cannot be sure of having something to live for unless we are willing to die for it.”
― Ernesto Che Guevara

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