5.56mm NATO vs .223 Remington

With the recent explosion and excitement over Modern Sporting Rifles (MSRs) as well as varmint hunting for fox, coyote, and prairie dogs we have lots of people asking what’s the difference between 5.56mm NATO and .223 Remington?

The .223 Remington caliber has been around since the late 1950s when it was first introduced as a military cartridge. The .223 Rem became popular with varmint hunters immediately because it outpaced the .222 Remington and .222 Remington Magnum in performance. It caught some serious popularity after the assault weapons ban in 1994.

Nosler Reloading Guide #7 – .223 Remington Dimensions

But then there is this other caliber everybody is talking about: the 5.56mm. Or was it 5.56 NATO? My neighbor calls it 5.56x45mm NATO… So who’s saying it right? The answer is: everybody is correct! Many names for the same military caliber. It was originally produced to be used in the M16 with 20″ barrels. These initial firearms used slower twist rates (the distance the bullet travels in the barrel before it makes one full revolution or spin) like 1 in 12″ or 1 in 14″. Today our military uses the M4 rifle with a significantly shorter barrel (14 .5″) and a much faster twist rate of 1 in 7″ allowing the rifles to accurately shoot heavier bullets (62 – 77 Grain bullets).

Hornady Handbook of Cartridge Reloading 9th Edition – 5.56mm NATO Dimensions

A lot of people see the two calibers as a Potatoe-Potato˜ argument where the difference between them doesn’t matter, but that’s just not true.

For those who buy boxed ammunition off of the shelf, you should know that a .223 Rem has a smaller chamber throat than a 5.56mm NATO. What this means is: only shoot .223 Rem ammo in a .223 Rem rifle. The bigger 5.56mm NATO caliber will damage your firearm. If you own a firearm chambered in 5.56mm NATO though since the 5.56mm NATO has a longer chamber throat you can safely shoot both .223 Rem and 5.56mm NATO. You may see less accuracy from .223 Rem in your 5.56mm NATO, but that’s just because it’s undersized; “it doesn’t fit.” It’s like trying to sprint a 100-Meter dash with shoes that are too big. You’ll finish the race, but it won’t be as graceful or as fast.

Sometimes I get asked what the difference is between the two chamber throat lengths; like what is the exact, numerical difference? People may ask this because they’re reloading and trying to hand-tailor ammunition for their personal firearm. When I give them a vague answer some people get mad, but that’s the best answer that can be provided. Between government, military, commercial, Match, National-Match, and other various barrels there is no consistent chamber throat length or industry-standard to abide by. In a vague, but transparent sense my answer always is: .223 Rem has a shorter chamber throat length while 5.56mm NATO has a longer chamber throat length.

A 2nd difference between 5.56mm NATO and .223 Rem is the pressures they can withstand. According to S.A.A.M.I. (Sporting Arms and Ammunition Manufacturers’ Institute) standards, the .223 Rem can be loaded to 55,000 P.S.I. (Pounds per Square Inch) while 5.56mm NATO can tolerate 60,000 P.S.I. of pressure.

What this can mean for the shooter is 5.56mm NATO has a shorter accurate service life since it is a more potent caliber. Simultaneously, 5.56mm NATO can effectively shoot heavier grain weight bullets making it more appealing to most people.

If you’re a reloader like myself, it is harder to find reloading data for 5.56mm NATO. I personally own 10 different reloading manuals and the only 2 that provided any data were Hornady and Nosler.

So if you’re contemplating owning or switching to a different caliber I hope this shed some light on this murky topic. I’ll leave you with these words from Patrick Sweeney taken from the Nosler Reloading Guide #7:

“Unlike the .223 Remington, the 5.56 is a no-hold’s-barred effort at making a varmint cartridge into a defense caliber.”