Written by Ballistic Technician Philip Mahin
Last time we were discussing why a 270 Winchester uses a .277” diameter bullet. Before we get too deep into this, we need to put a face with the name. When you hear the term 270 caliber (or 30 cal for that matter), this is referring to the firearms bore diameter. This shouldn’t be confused with groove diameter and I’ll show you why.
The groove diameter will be .277” but the bore is .270”. This puts an engraving into the bullet and grips it by the rifling as it travels down the barrel making it spin in flight. Now the 270 Winchester was an easy one. What about the 7.62×51? It can also be called 7.62 NATO or 308 Winchester depending on what chamber you have. The 7.62mm translates to .300” or 30 caliber for a bore diameter and so .308” translates to 7.82mm for a groove diameter. That is also the way Lazzeroni named the 7.82 Warbird. It still uses a .308” bullet but it is a name different enough to not get confused with anything else. But what about the x51 on the NATO cartridge? That actually corresponds to the length of the case in mm and would equal 2.015”. The 30-06 cartridge would look like 7.62×63 in military form and there are others like the 5.56 / 223 that are confused quite often. By the way, the 223 Remington cartridge uses a .224” diameter bullet. The .223” diameter bullets we sell are primarily used in the 22 Hornet cartridge chambered in older guns that actually had a .223” diameter groove. More to come . . .