According to my list of bore diameters, 10 is .775" - 8 is .835".

Modern rifles are measured for size to the bottom of the grooves. Black powder guns, ML's as well as Breech Loaders were usually measured to the bore diameter, not the groove diameter. Many people say their bore is .xxx" but they are meaning the groove diamter - it is confusing. This makes it difficult to address proper ball, patch or bullet diameters for use in their barrels.

I've a friend with a, 11 bore(.751") rifle as in .75 cal. English Sporting Rifle, period 1855 - Purdey style. It is a Muzzle Loading Rifle and the barrel is indeed an 11 bore at .750"(close enough) as that is what it measures. Unfortunately for him (another story) the rifling is .025" deep - PER SIDE, giving this rifle a groove diameter of .800". How many people would call this gun a 9 bore, (9 bore is .803") - but wrong - it is a .75 cal barrel so it is an 11 bore fur use with round balls and patches. If he shot groove diameter conicals in it, it would indeed, be a 9 bore - it can be confusing.

When shooting round balls with a patch - we measure a gun's calibre by the bore diameter as that measurement effects the size of the ball that can be used, whether it is a rifle or smoothbore also effects the size of the ball that can be used.

When shooting a bullet, the groove diameter effects the diameter of the bullet, so we use groove diameter for addressing it's size.

20" of twist is a lot fast for a 10 bore, let alone an 8. 20"of twist is for 1" to 1 1/2" long bullets in a .45 rifle. In order to need or utilize that much twist in a 10 bore, you would need a bullet that was 2 to 3 calibres long. How heavy is a 1 1/2" to 2 1/4" long bullet of .750"? Who could stand the recoil - what gun could - ridiculous.

50" to 70" would work for a short conical or the original type in 10 to 8 bore and yet, still work reasonably well with round ball if one desired.