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I see your point and you are correct. They try to confuse you. I never actually did the conversion. I have just went on what the gun world calls it, what I was taught in gun smith school. Now we need to get into the diameter of lands compared to the grooves. :p
 

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He-he, Good one!! :)(y)

Forgot - 0.284 inch = 7.213 6 millimeter

Motorcycle companies do a similar thing with their numbering & CCs/cu.in.

I'm sure you know this from gunsmith school. But, for others......FYI - Land to land diameter is the 'bore'. Groove to groove diameter is the 'bullet diameter'.

PS Shotgun gauge is also called bore in the UK/Europe. There you might see shotgun shells labeled 12 bore. Just learned that recently.
 
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The bore of a shotgun measured in gauge is an interesting measurement.
The “gauge” is the number of lead balls in the diameter of said bore to equal 1 pound.
So a 12 gauge takes 12 lead balls with a diameter of that bore (0.725”) to equal 1 pound. So that’s why the lower the gauge, the bigger the bore because it takes less lead balls of larger diameter to equal 1 pound.
The only exception is the .410, which is the actual diameter of the bore in inches. This equals 67 1/2 gauge.
Sorry for getting off topic, but I’ve found this is a little known fact and wanted to share.
 
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Now what chamber is it? :p

.224 Valkyrie is based on the 6.8 SPC case, so the 6.8 BCG fits the Valkyerie, same for mags. Which has made mags that fit both and BCGs more available I've seen over the years.
 

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I am pretty sure I am correct on the diameter of all of them. You stated " A 6.8/.277 (Actually 7.0mm) bullet". That is not correct. A 6.8 is a .277 diameter bullet, a 7mm is a .284. That was the point I was trying to make. The .270 Winchester is the same diameter as the 6.8 SPC. What is this .270 Winchester "creation mystery" you speak of? It is simply a 30 06 case necked down to .277. No mystery that I know of.
Your stated bullet diameters are correct. The .30-06 case is slightly shorter than the .270 Winchester (2.494 vs 2.56), the difference being entirely in the longer neck on the .270. I had read recently that the .270 was developed from the .30-03 Springfield cartridge case (the direct parent of the -06), which was .070" longer than the .30-06. Maybe this is the "creation mystery" that was being referred to? I know it was always a question mark to me, so......
 
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