Rifle Calibre Perplextion!

Question
Hello!
My name is Matt and I have been a British sniper for 7 years now. I had my world crash around me when lately I worked out that the 5.56mm round is actually 0.219' and a 7.62mm is 0.300'.... instead of .223 and .308, that I always asumed was the imperial convertion.

I generally use the 8.59mm and 12.7mm which correctly convert to both there imperial diameters, .338' and .50' respectively.

What is the correct diameter of these two rounds? And why the descrepency?

Yours Matt

Answer
Matthew,

According to the information I could locate, the .338 Lapua uses a bullet right on .338 inches, or 8.59mm. The 12.7mm or .50 Browning Machinegun cartridge actually runs .51 or .511 inches.

As for the discrepency, I'm not exactly sure. Oftentimes a cartridge name ends up sticking before the final dimensions of the cartridge are worked out. I'm not sure of the development history of the 5.56 NATO or 7.62 NATO, so I'm not exactly sure how they came to be called a number that's slightly different than its caliber.

I wish I could have been more helpful,

Kyle