Measuring the case diameter at the web.

[MVW]

Gents I was wondering if there is a particular recommendation for measuring the diameter of a 308 case at the web. I have a mic, but my concern is that small changes up or down the case could lead to an erroneous measurement.

Thanks, Mike

 

[gotcha]

Hi Mike, I think I'd measure the depth of the case w/ the little depth gauge extension on my calipers. Measure several to get an average to eliminate measuring possible burrs on inner flash hole. Subtract that measurement from the case length. this will tell you how far above the base the web actually extends. Use a caliper (not a micrometer) to measure the dia. of the web. Hold the case just tight enough that the weight of the case doesn't allow it to slip position when hanging free & vertical. Simply stated, measure as close to the extractor groove as possible to avoid case wall bulge or sizing die influence. Hope this helps

 

[kelbro]

Are you using the measurement to look for pressure signs? I use a blade mic. Just find the largest diameter.

 

[MVW]

Kelbro, yes I am using it to look for pressure signs. Thanks. I'll look for a blade mic.

Mike

 

[robbor]

if you want to get a real accurate reading find a machine shop or college that has a machining class and ask to use their optical comparitor. Put it on the base and come up .100" then measure across, new and fired. Other than that i dont know of a way to get a real accurate measurement. Maybe use a bullet runout gauge and set it to zero on new brass and then measure the fired case expansion.

 

[BoydAllen]

I think that the best way to get consistent measurements of expansion of the web is to look around on Ebay and find a blade mic. that measures to .0001 and measure the diameter at the bottom of the extractor groove. With experience, you will learn to interpret the readings that you get. You should make a mark across the diameter of the head, with a permanent marker and measure in line with the mark each time. Because heads work harden, you cannot compare the expansion of an old case with a new one. I think that the best readings are probably on the second firing of a new case. On a belted case, you can mark the head and measure the belt, with a regular mic.

 

[MVW]

Thanks Boyd. I'll see what I can find. Mike

 

[wapiti25]

Yes, use a blade mic. You should look for about .0001-.0002, .001 is a lot of expansion.

 

[kelbro]

Agree with above. My post was lacking that critical bit of info. .0001 blade mic. I use the extractor groove AND the expansion ring measurements.

A good baseline is the expansion of factory brass. I seldom purchase factory ammo but over the years I have seen .0001-.0003 and now I just use that to tell me when to stop.

 

[FroggyOne2]

I read years ago, that if your expansion reaches .003, your at exssesive pressure! This I feel is the only way to see if your at pressure or not, well if your blowing primers or leaking around them on new cases your at pressure too! But what I am getting at is, that with a lot of modern actions with their polished bolt faces and using small firing pins that fit perfectly in the hole, harder cup primers, allow a shooter to run more pressure. In theory that is, but your still running pressure. It is just that the signs to it are more latent.

Once heard about a guy that was shooting a 223 Palma gun, built on a Bat action, while at the Spirit of America match. The guy was running so much pressure that the bolt came apart, meaning that the firing pin seperated from the bolt body and impaled itself in the shooters face.

So remember, to always check your cases at the web for pressure, it is the best way to make sure that your within reason on the load that your using.