Bullet diameter

[itchyTF]

Am I missing something?? I don't have a mic but do have a Mitutoyo dial caliper with .001 graduations. Last night I was measuring some 6mm bullets like 105r A-Max, 75gr Sierra HP and some Bergers. I was reading .242 for a diameter! A solid .001 less than groove diameter. I didn't check last night but I could swear that when I measured .224 bullets in the distant past that they measured .224. I should have checked to see if I'm loosing my mind. Guess I'll do it tonight.

 

[spclark]

If more than one brand and style of bullet is reading undersize, I'd doubt the veracity of the measuring instrument I'm using.

Lapua bullets are often said to be on the smaller side of a given caliber but in my experience the other brands mentioned are always right on.

If your Mitutoyo is engraved to only 0.001" then it lacks sufficient tolerance to measure bullets with any accuracy. The digital styles are better in that regard, but none will serve you better than a dedicated micrometer for measuring dimensions smaller than 0.001".

 

[TonyR]

You need a micrometer graduated in 0.0001 inches. That said, the diameters can vary by several ten thousandths between manufacturers and even within the same lot from some makers. It also matters where the measurement is taken on the shank. I've never tested one size versus another for accuracy but there are differences.

 

[RonAKA]

Yes, you need a micrometer that reads to the tenth of a thou. It does not have to be digital or expensive. Non digital ones have a vernier scale that allow you to read to the 10th or even a bit better -- $20 for a made in China version.

Measuring bullets is not as simple as it looks. Many flat base bullets like Berger have a narrow pressure ring at the base that measures about 0.0004" larger than the body. Other bullets have a taper in the body, so it matters where you measure.

For the 75 grain HP Sierra I measured some a while back and they were extremely consistent. There was no pressure ring I could measure and the body on every one measured was the same at .2426". Some 68 grain Bergers measured 0.2427" in the body, and 0.24316 (average) at the pressure ring.