Brass stretching consultation please!

[chuckstur]

I blew a primer in my new 6mm CM with a moderate load so I bought a go gauge and case gauge. to do some troubleshooting The bolt closes easily on the go gauge but will not close when I put a piece of clear tape on it. The go gauge sits halfway down the 0.008" deep groove on my case gauge, as does sized brass. So far so good, me thinks.

When I drop the fired brass into the case gauge I find it has stretched about 0.017" longer than the go gauge. I never noticed that the bolt was hard to open. Is this normal? Thanks,
Charlie

 

[Sheldon N]

Something seems wrong with your measurements, probably in how the brass fits into your case gauge on the shoulder angle or some other interference point. Your chamber is confirmed as being in spec, closes on "go" and won't close on "go + tape" which is about +0.004". If your chamber is in spec, there's no way it could produce fired brass that was 0.017" longer than the go gauge.

My recommendation would be to get a set of the Hornady headspace comparator gauges that allow you to measure a specific line on the case shoulder. At that point you don't need the go-gauge, all you look at is the measurement of fired (fully expanded) brass and compare it to either virgin brass or sized brass to make sure you aren't shooting rounds that have excessive headspace relative to your chamber. Then when you are setting up your sizing die you can fine tune your die so that it is producing brass that is 0.002" shorter at the shoulder than fired brass.

 

[Huskydriver]

I concur with sheldon... case length gauges are old school technology from when the school of thought on reloading was full length size or neck size only and give you no real clear measurement of what is actually going on other than it fits in the case gauge and should be go. In a day when bushing FL sizing dies is the new norm using a bump sizing technique I don't see the point in a case gauge when the object of resizing is to size your brass everytime to fit your specific chamber -.001,.002 and not a sammi spec cut case gauge. I suspect the brass is being way overworked based on the chamber cut of the case gauge. Get yourself a set of comparators and measure your once fired brass against what you are sizing your brass to.

 

[chuckstur]

I have been designing machinery for 40 years but could not reconcile my measurements either. It now makes sense. Thanks a bunch folks.

 

[bax]

I use the RCBS case gauges. They give me adequate precision and repeatable results.

If would guess the case gauge has a tighter neck or assumes a full-length-sized case with different shoulder-to-neck radius than your chamber. The fired case sticks out because the contact point is on the radius and not on the shoulder. But, if you are in the business you would have thought of that so probably a stupid idea.

 

[chuckstur]

Every internet post will attract at least one snippy reply.