.243 diameter question

[Dawsie]

Is a .243 rifle the internal diameter of the barrel or the outer diameter of the bullet, or are they the same thing?

 

[Laurie]

6mm calibre bullets are nominally 0.243" (6.2mm) diameter. The barrel bore diameter (ie what it starts bored to before rifling grooves are cut or swaged into it and therefore forms the lands on the finished article) is usually 0.237" diameter, and the grooves are usually cut on a match barrel to the bullet diameter, ie 0.2430" from the bottom of one land to the bottom of one opposite, so in this case the lands are 0.003" depth. The barrel dimensions can vary either through customer specification or production tolerances, usually only by a few tenths of a thou' but in same cases by a full thou' and bullet can also be either slightly under or oversize depending on type and manufacturer.

Modern cartridge terminology as in 243 Win tends to use the bullet / groove diameter, but the traditional English riflemaker's / designers practice was to use the bore / lands diameter so we called 7mm cartridges .275s for instance instead of .284s.

 

[8x57]

Reference what Laurie says above regarding bore size prior to rifling and the traditional way that British makers measured barrels, its often been argued that technically rifles chambered in .243 win aren't deer legal for the larger species of deer because they are less than the required .240 calibre.

Now that could throw a real spanner in the works if someone was to successfully put that argument forward in certain circles.

Just think of how many thousands of deer could have been shot illegally. ​