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 NameCommon Metric NameBullet Diameter
.174.5mm.172″ (4.32mm)
.2045.2mm.204″ (5.2mm)
.2235.56mm.224″ (5.7mm)
.22 (lr)5.6mm.223″ (5.66mm)
.22 (not lr)5.6mm.224″ (5.7mm)
.2436mm.243″ (6.17mm)
.2576.53mm.257″ (6.53mm)
.2606.5mm.264″ (6.71mm)
.2706.8mm.277″ (7.04mm)
.2807mm.284″ (7.21mm)
.307.62mm.308″ (7.82mm)
.3258mm3.23″ (8.2mm)
.338.6mm.338″ (8.59mm)
.3669.3mm.366″ (9.3mm)
.3759.5mm.375″ (9.53mm)
.41610.6mm.416″ (10.57mm)
.5013mm.51″ (12.95mm)

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 CartridgeBullet Diameter in InchesBullet Diameter in MM
.22 LR.223″5.66mm
.357 Magnum.357″9.1mm
.380 ACP.355″9mm
.38 Special.357″9.1mm
.40 S&W.40″10mm
.44 Magnum.429″10.9mm
.45 ACP.452″11.5mm
.50 AE.5012.7mm

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

CartridgeCaliberBullet Diameter (mm)
.17 HMR0.1724.37
.17 Hornet0.1724.37
.17 WSM0.1724.37
.204 Ruger0.2045.18
.22 Creedmoor0.2245.69
.22 Hornet0.2245.69
.22 Nosler0.2245.69
.22 WMR0.2245.69
.22-250 Remington0.2245.69
.220 Swift0.2245.69
.222 Remington0.2245.69
.223 / 5.560.2245.69
.224 Valkyrie0.2245.69
.240 Weatherby Magnum0.2436.17
.243 Winchester0.2436.17
6mm BR0.2436.17
6mm Creedmoor0.2436.17
6mm Remington0.2436.17
.25-06 Remington0.2576.53
.257 Weatherby Magnum0.2576.53
.26 Nosler0.2646.71
.260 Remington0.2646.71
.264 Winchester Magnum0.2646.71
6.5 Creedmoor0.2646.71
6.5 Grendel0.2646.71
6.5 PRC0.2646.71
6.5 Weatherby RPM0.2646.71
6.5 x 55 Swedish Mauser0.2646.71
6.5-284 Norma Match0.2646.71
6.5-300 Weatherby Magnum0.2646.71
.27 Nosler0.2777.04
.270 Weatherby Magnum0.2777.04
.270 Winchester0.2777.04
.270 WSM0.2777.04
6.8 Remington SPC0.2777.04
6.8 Western0.2777.04
.28 Nosler0.2847.21
.280 Ackley Improved0.2847.21
.280 Remington0.2847.21
7mm Mauser0.2847.21
7mm Rem Mag0.2847.21
7 SAUM0.2847.21
7 STW0.2847.21
7mm Weatherby Magnum0.2847.21
7 WSM0.2847.21
7mm-08 Remington0.2847.21
.30 Nosler0.3087.82
.30-06 Springfield0.3087.82
.30-30 Winchester0.3087.8232
.30-378 Weatherby Magnum0.3087.82
.300 Blackout0.3087.82
.300 PRC0.3087.82
.300 RUM0.3087.82
.300 Ruger (RCM)0.3087.82
.300 Weatherby Magnum0.3087.82
.300 Winchester Magnum0.3087.82
.300 WSM0.3087.82
.308 Winchester0.3087.82
7.62 x 39mm0.3087.82
.325 WSM0.3238.20
.33 Nosler0.3388.59
.338 Federal0.3388.59
.338 Lapua Magnum0.3388.59
.338 RUM0.3388.59
.338 Win Mag0.3388.59
.338-378 Weatherby Magnum0.3388.59
.340 Weatherby Magnum0.3388.59
.35 Whelen0.3589.09
9.3 x 62mm Mauser0.3669.30
.375 H&H Magnum0.3759.53
.375 Ruger0.3759.53
.378 Weatherby Magnum0.3759.53
.350 Legend0.3789.60
.416 Remington Magnum0.41610.57
.416 Rigby0.41610.57
.416 Ruger0.41610.57
.444 Marlin0.42910.90
.450 Bushmaster0.45211.48
.45-70 Govt0.45811.63
.458 Win Mag0.45811.63
.50 BMG0.51012.95