Many shooters are confused because there are two different systems for measuring the bore diameter for a cartridge: (1) the caliber system measuring the diameter of the bore of the barrel in inches, and (2) the same measurement, but in millimeters. This page will explain the problem as clearly as possible.
The manufacturers of ammunition have done us no favors in making this easy to understand, but I’ve put together this handy resource so you can quickly see the equivalent naming conventions between mm and inches (caliber).
Rifle Cartridge Caliber to Metric Conversion Table
|Rifle Caliber Name||Common Metric Name||Bullet Diameter|
|.22 (lr)||5.6mm||.223″ (5.66mm)|
|.22 (not lr)||5.6mm||.224″ (5.7mm)|
If you’re like me, looking at the table above starts to make your head spin. There are some things in that table that just don’t seem right. That’s because the naming conventions for many of the most popular cartridges bend the rules of measuring in order to come up with a nice sounding name.
For example, the good old .308 Winchester is so named because the bullet is .308″ in diameter. Converted to metric, that’s 7.82mm, but most people would refer to it as a 7.62 because of its military distinction. The difference is that one measurement is the distance between the grooves (the cut out portions of the rifling in the barrel, and another measurement uses the measurement to the lands (the raised portion of the rifling). To make matters worse, the .308 is commonly referred to as a .30 caliber cartridge, which isn’t actually correct at all.
As you can see, the naming conventions of cartridges is complicated because ammo manufacturers like using clean, even numbers and to ignore the actual measurements.
Because the table above has to make some generalizations about particular cartridges, I’ve included the table below which lists each rifle cartridge separately so you can see the specific bullet diameter in inches and its exact metric conversion.
Common Handgun Calibers Converted to MM and Inches
|Handgun Cartridge||Bullet Diameter in Inches||Bullet Diameter in MM|
Fortunately, understanding the conversions between metric and imperial for handgun calibers is quite a bit easier because there are far fewer common pistol cartridges than there are rifle cartridges.
For the most part, handgun calibers are either 9, 10, 11, or 12mm.
List of Rifle Cartridges and Their Bullet Diameters in Inches and MM
|Cartridge||Caliber||Bullet Diameter (mm)|
|.223 / 5.56||0.224||5.69|
|.240 Weatherby Magnum||0.243||6.17|
|.257 Weatherby Magnum||0.257||6.53|
|.264 Winchester Magnum||0.264||6.71|
|6.5 Weatherby RPM||0.264||6.71|
|6.5 x 55 Swedish Mauser||0.264||6.71|
|6.5-284 Norma Match||0.264||6.71|
|6.5-300 Weatherby Magnum||0.264||6.71|
|.270 Weatherby Magnum||0.277||7.04|
|6.8 Remington SPC||0.277||7.04|
|.280 Ackley Improved||0.284||7.21|
|7mm Rem Mag||0.284||7.21|
|7mm Weatherby Magnum||0.284||7.21|
|.30-378 Weatherby Magnum||0.308||7.82|
|.300 Ruger (RCM)||0.308||7.82|
|.300 Weatherby Magnum||0.308||7.82|
|.300 Winchester Magnum||0.308||7.82|
|7.62 x 39mm||0.308||7.82|
|.338 Lapua Magnum||0.338||8.59|
|.338 Win Mag||0.338||8.59|
|.338-378 Weatherby Magnum||0.338||8.59|
|.340 Weatherby Magnum||0.338||8.59|
|9.3 x 62mm Mauser||0.366||9.30|
|.375 H&H Magnum||0.375||9.53|
|.378 Weatherby Magnum||0.375||9.53|
|.416 Remington Magnum||0.416||10.57|
|.458 Win Mag||0.458||11.63|