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Coup De Grace measurement... how?

I'm sure you've done many more cone breeches than I can ever even imagine doing, so maybe you have some insight into how to reliably gauge that 0.085 distance. There's no square surface to mic against. I think that's why they supply the gauge ball drawing as well. The ball will seat solidly against the cone and can then be measured by conventional mics.

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(full disclosure, I just set that barrel on the bench up for photo)
I agree gage balls are a very precise way to measure many things including diameters in chambers. I know of one inspection report for 50 cal barrels that requires 10 different measurements using gage balls. But I think a lot of people get to hung up on bolt nose clearance. The bolt needs to fit close enough to be safe. In this case it shows .147" exposed case head. That could easily be .167".
In this case I would set my compound on 30' and as soon as the tool touched I would zero out my DRO or travel indicator. I'd make it .157" deep and still sleep soundly at night.
Another way would be to make cone shaped gage say with a 30' nose. Use it like a HS gage. Get a measurement prior to cutting the CB and then do the math as you get deeper.
Here are two gages I made probably 35 years ago when I was doing a lot of SR BR work. One 25' and one 30'.

20231012_090101.jpg
 
I do a lot of pre fits. But I have also had a decent amount of the actions that are pre fit compatible that come though that were not in spec. Recently had one that should be a .150" Remington nose that was .155". I cut for .007" clearance but ended up with .002". That rifle would not have shot well that way so I fixed the bolt. Until I have seen a lot of one action my trust is not there yet. So far never seen one out of spec to be dangerous so thats good.
 
I was bored so I went ahead and did the CAD on this and, given the ARC dimensions posted earlier in this thread, it looks like a 1 inch gauge ball will just make it with a .308 family chamber--by about 15 thou, so with a magnum chamber and you'll need the 1.25 ball which is larger than the threads and so a special fixture would be needed.

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