Do any of our continental bretheren know why a 6.5mm bullet is actualy .264" (6.7mm) and not .256" as it would be if you converted from mm to inches. I allways wondered how it happened. Maybe they thought 6.5 was a catchy name for .264 which seemed a good size, didn't stop them coming up with 9.3 or 10.75 though. Also don't you just love the cheek of Rigby in taking advantage of this disparity in the 7mm and calling the 7x57 a .275 Rigby which is what 7mm would be if it truly were 7mm and not 7.2mm. Can anyone shed any light on how these differences arose?

Traditionally, European metric caliber are referred to by "bore" size -- the distance between the tops of the riflings, while American (inch) calibers are usually referenced by groove size -- the distance between the bottoms of the grooves between riflings. The height of the rifling is usually about .004 inches, for a differnce of about .008 inches in total diameter.

7mm = .276 inch = .284 inch groove and bullet.

6.5 mm = .256 inch = .264 inch groove diameter and bullet diameter.

6mm = .236 inch = .243 bullet (as bore goes down, usually also does the depth of rifling)

The proprietary Lazzeroni line has reversed this convention and labels their metric cartridges by actual bullet diameter (which really confuses us older guys).

1894,

By the same reason a 300 Winchester Magnum bullet does actually have a .308" diameter.

There is nothing to add to the very enlightning explanation given by Stonecreek, except for the fact that manufacturers, regardless of their nationality, follow no rules whatsoever. Take Winchester, for example, they named their 270 by its bore diameter whilst following the opposite rule with their 308.

Best regards.

Montero

Stonecreek,

Many thanks, this has allways puzzled me.

But Brits have always called the 6.5 a .256, and the 7mm a .276. Hadn't you noticed?

Gentlemen,

Lazzeroni is the first one to get it correct. For the hunter-loader the diameter across the lands is immaterial. Bore diameter, which matches bullet diameter is far more relavant.

A .243" bore-bullet is actually 6.17 mm.
A .264" bore-bullet is actually 6.71 mm.
A .284" bore-bullet is actually 7.21 mm.
A .308" bore-bullet is actually 7.82 mm.
A .322" bore-bullet is actually 8.20 mm.
A .366" bore-bullet is actually 9.30 mm.
A .375" bore-bullet is actually 9.53 mm.
A .416" bore-bullet is actually 10.57 mm.

Good Hunting !

Slamfire,

We only called the mannlicher .256 and even that is called 6.5x54schonaur by many and the Rigby was .275

[This message has been edited by 1894 (edited 06-18-2001).]

1894. I think the .276 was Westley-Richards version of the 7x57.
Paul B.