Tobes, the generic calibre size usually refers to the BORE size, not the diameter of the grooves. 7mm cal is 7mm because the bore size (measured with a bore pilot bushing) is usually 0.275" to 0.277" (7mm) whereas the groove diameter is nominally 0.284". The bullets are usually sized to match this larger diameter.
Same thing with 6.5cal. The bore size is 0.255" to 0.256" (6.5mm) and the groove diameter is 0.264" .
Does that explain it ok?
D.
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