Easy Conversions from MM to Caliber to Gauge

We’re a stubborn nation, there’s no doubt about that. That’s not always a negative thing however. That stubbornness has brought and still brings us greatness. Our stubbornness to refuse anything but complete freedom and when it’s necessary to fight our guts out for it. Our stubbornness to live free while under imperial rule of Great Britain that was the birth of our freedom. Our stubbornness has built this great country into what it is today.

But… at times it can be a headache.

I remember in grade school a lame attempt to catch us up to the modern world in one specific area we were lacking. Measurements of course!

The battle between the Imperial System and the Metric System. And we as gun owners who value the condition of our guns have paid the price.

Why is finding the correct gun cleaning brushes so complicated? It’s not really, we just need to know how to choose the correct size brush or bore snake in order make sure we are getting a proper fit.

Do I buy a .270 caliber brush or 7mm? What about a 10mm? Does a 28 gauge convert into anything else that’s useful besides a 28 gauge?

All worthy questions. Let’s slow down and back up a bit. If we convert everything to plain old decimals of an inch it’s easy for us to see how all of these measurements relate.

First let’s simplify “caliber”. Caliber is the diameter of a round or internal diameter of a hollow cylinder usually expressed in hundredths or thousandths of an inch, according to Webster’s Dictionary. A bullet’s diameter dang well better match the inside of your gun’s bore or we’re going to have problems right?

Luckily this is how almost all rounds are labeled. A .270 caliber rifle bore has an inside diameter of 0.270”. A .45 cal handgun has an inside diameter that measures 0.45”. The various .30 cal (30-06, 300 RUM, .308) have the measurement of 0.30”.

We can use that information and the chart provided for you below to easily convert between the important caliber, mm and gauges. You’ll notice that there is some small discrepancies in measurements. Sage & Braker oversized their bore cleaning kits by roughly 0.03” in order to ensure full bore contact, so close is good enough.

Here’s the breakdown.