micrometer readings of a slug

[PerpetualStudent]

Hello,

I've been doing this in the Johnny Cash manner (one piece at a time). So I've been a member for a while but only just slugged my barrel.I drove a pure lead .360 roundball through a .357 lever action barrel and measured it with a micrometer. I've never used a micrometer before so I'm aware the variance might be me.

I got 2 groups of measurements. One group average .3505 and the other group .3576 they're pretty tight around each other with a max difference between measurements of 0.0005 and 0.0007 respectively. With that in mind I've got 2 questions:

1. Should I just look at the average of the measurements as the guide off which I buy my sizer? Or if the numbers are that much different is the problem me, slug it again, make sure to measure perfectly?

2. I'm confident the larger measurement is the widest area in the bore (so measures the bottom of the land to bottom of the land). But I'm not quite sure if the smaller measurement is a land to groove or groove to groove measurement. Is there something obvious I'm missing to tell?

[country gent]

Measure some "known" items with your mic, IE jacketed bullets are very close to what the box says, dowel pins are good, drill bits, any thing that is a known size. See how close you are there and adjust your feel to this.

If you want the rifling to engage the slug you need to size to the larger dia usually +.001 if possible, so a .358-359 sizing die. The chamber may also limit how big you can go as a bullet to big in the case wont allow it to chamber easily. To large a loaded round neck dia.

[mdi]

For some, getting the "feel" of a micrometer can take practice. One hint I used when teaching micrometer use is to use the ratchet (the small part sticking out of the end of the thimble). Find a known diameter part and measure using the ratchet for "tightness" of closing on the part. Then measure the part just using the thimble, being very gentile and see if you two measurements are the same. If your second measurement is smaller than the first, "standard" measurement then perhaps you are closing on the part too tightly (I has one apprentice that closed the mic down on a part like it was a C clamp!). Practice...

For measuring a barrel slug you are interested in the large diameter, the groove diameter. I lightly close the mic on the "fattest" part of the slug, and spin to another fat part several times for each slug and I normally slug each barrel 3 times, just to make sure.

In your case I would buy a sizing die .002" larger than your largest measurement, the groove diameter...

BTW the measurements you quoted, .0005" and .0007" are in ten thousandths of an inch, and unnecessarily accurate. Measurements of thousandths of an inch are enough, three digits beyond the decimal point. I know being new to precise measuring an extra zero ain't no big deal, and getting used to the terminology, just sayin'.

[PerpetualStudent]

Ok. I will put some practice in on some known diameters. I used the ratchet not the spindle to close on it so I'll also experiment with the two different types.

The measurement differences with the ten thousandths are maximum differences in the measurements I got. So for bore diameter the smallest was 0.3573 and the largest .3580, all except the max were 0.357x, so if all that matters is the thousandths then maybe I should just say it's high between .357 and .358 and get a sizer for .359
I also know I have a tendency to overthinking and being overly precise. Hazards of being raised by an engineer.

[mdi]

Bullet sizing to the .0001" variation is unnecessary. Even thinking in less than one thousandth is often unnecessarily accurate. The differences you quotes are 2/10,000th inch. A human hair averages .002" or two thousandths of an inch (2/1,000").

And there is the "depends clause"; I once machined some parts out of titanium and some out of stainless steel at a shop in So. CA. The parts had a tolerance of + .001", - .000". All the parts , because of the tight tolerance were checked 100%. When the parts were inspected at the customer's location, in Alaska, all were rejected as under size. Ambient temp difference of 50 degrees "shrank" the parts. So, depending on the alloy, the ambient temp (very little but still a factor), method of measuring and measuring tool used, .001" variation is acceptable measuring bullets and barrel slugs...

[Wayne Smith]

Learning to use a micrometer is one good skill. The base measurement you need is the large diameter in your barrel. You want to go a couple thousands over that.