With so many calibers out there making it difficult to know which is better, it becomes more confusing when some calibers are stated in U.S. Standard Imperial inch terms, like the 308 Win, or in metric terms, like the 7.62mm. In fact, the 308 Win is a 30-caliber, and the 7.62mm is also a 30-caliber. You should also know that the bore size stated on a firearm is actually smaller than the bullet that gets fired through it, and many calibers are “somewhat” to “very” misstated. For example, the 38 Special is actually a .357” diameter, the 25-06 Rem is .257” diameter, the 260 Rem is .264” diameter, the 280 Rem is .284” diameter, the 30-06 is .308” diameter, the 325 WSM is .323” diameter, the 350 Rem Mag is .358” diameter, the 223 Rem is .224” diameter, and so on.
 

You must first understand that most U.S. standard calibers are stated in inches, while metric calibers are stated in millimeters (mm). We see a lot of calibers like 5.56mm, 6.5mm, 6.8mm, 7mm, 7.62mm, 8mm, 9mm, 10mm, etc.  So, a 30-caliber bullet is actually 0.30” diameter, but it’s also a 7.62mm diameter. On a side note (and to reduce confusion), 7.62mm refers to the internal diameter of the barrel at the lands, so the actual bullet caliber is 7.82mm, which we will later find is .308” diameter. So how do we convert metric to U.S. standard, and standard to metric?
 

Let’s break-up 1” (1-inch) into 100 smaller units, each 1/100th of an inch. So a 30 caliber is 30 units of 100 units (0.30"), and a 50 caliber is 50 units of 100 units, meaning a 50-cal is a 1/2 inch bullet. We can state a 30-caliber as 0.30”, or better yet, drop the decimal and call it a 30-caliber. To better understand the relationship between similar calibers stated in different measures, you have to remember from a high school math class that 1” also equals 25.4 mm.  I wasn't much into the metric system before now.
 

Again, let's think of 1” in terms of the number 100; or 100 units. Now, divide those 100 units by 25.4 mm metric units, and we come up with 100/25.4 = 3.937. This is the approximate multiplier we need to convert a mm caliber to a U.S. inch caliber, or vice versa. So, a 7.62mm is actually a 30-caliber, since 7.62 x 3.937 = 30. What if we want to convert a 30-caliber to mm? Again, let's get rid of the decimal. Just divide 30 by 3.937, so 30/3.937 = 7.62 mm. To make life easier we can usually say 4 instead of 3.937, and the rounding error won’t usually be too much. So, 7.62 x 4 = 30.5, which is close enough. Likewise, 30/4 = 7.5mm. Recall, we were expecting 30-caliber and 7.62mm, instead of .305" and 7.5mm that we got from rounding error.  So why is a 7.62 a 308-caliber?  Recall, the actual bullet diameter of a 7.62 bullet is 7.64 (more confusion).  Just multiply 7.64 x 3.937 and we get 308.
 

Now let’s try this with a 7mm Rem Mag. What is the caliber equivalent? Well, let's multiply; 7 x 4 = 28 caliber (.28"). In fact, the 7mm Rem Mag is the same size bullet as what we know as the 280 Rem and 284 Win, which are both actually .284”.   Just so happens the 280 Rem is a 7mm bullet in a modified 30-06 cartridge.  Just so you know (and unrelated to this article), the 25-06 is a 25-caliber bullet in a 30-06 case.  What other calibers share the 30-06 case? The 270 Win, the 338-06, and the 35 Whelen.  Want to know what calibers share the 308 case?  They include the 243 Win, 260 Rem, 7mm-08 Rem, 338 Federal, and the 358 Win.
 

Back on topic.  Can you figure out why AR15 style rifles come chambered in the same caliber but with two names; the 223 Rem and 5.56mm? Recall that a 223 is actually .224” (again, more confusion). The math says 5.56 x 4 = 224. Finally, how about the 6.5mm caliber. Turns out to be almost the same as a 257-inch caliber. The .257" bullet size is the same for the 25-06 Rem, 25 WSSM, 257 Roberts, and the 257 Weatherby Mag; who knew? So, multiply a metric (mm) caliber by either 3.937 or 4, and divide an inch caliber by 3.937 or 4. It’s usually close enough.
 

Something else that adds to confusion is when a caliber is stated like a 25-06, 30-06, 6.5mm x 284, 7.62 x 39 mm, 7.62 x 54 mm, and so on. Let’s start with the 30-06. The “06” part just tells us the cartridge was a 30 caliber adopted for military use in 1906. Going back to the 25-06 Rem, it’s a 25-caliber bullet (actually 257) in a necked-down 30-06 case. The 338-06 is a 338 caliber in a necked-up 30-06 case. For the 6.5mm x 284, the 6.5mm bullet is in a necked-down 284 Win cartridge. Now, when the caliber ends in “x  ##mm”, like 7.62 x 54mm, the last number (##) is the cartridge length, in mm. So, the 7.62 x 39mm and the 7.62 x 54mm are the same size bullets, but in different length cartridges. The former is the AK47 round and the latter is the M14 or AR10 round.
 

The point of the article is simply to help make the conversion and understand the cartridge. A good discussion can be found at the links below, and it also includes a chart showing U.S. Standard (aka Imperial) versus the metric mm equivalent, and many of the cartridges that fall in that family. Happy exploring!

Link: https://en.wikipedia.org/wiki/Caliber
Link: https://en.wikipedia.org/wiki/List_of_cartridges_by_caliber