What’s the Difference Between .223 and .308? An In-Depth Guide

When it comes to rifle ammo, two calibers are king. Notably, the smaller, faster .223, also known as the “AR-15 caliber,” or the larger, hard-hitting .308. Both of these calibers have some advantages, but which one is the best choice for building an AR-15 around? In this post, we’ll look at the similarities and differences between the .223 and .308 calibers and discover which is the best for an AR-15 barrel.

What is .223: The AR-15 Caliber

The .223 Remington, known commonly as “.223,” has become the most ubiquitous round in America; according to sources, civilians purchase more .223 ammo than any other caliber. But that statistic is most likely due to the US Army adopting the cartridge in 1963 to feed what would become its workhorse for a half-century: the M16.

The .223 bullet is light and fast, their muzzle velocity clocks in at over 3,000-feet-per-second and 3,600-feet-per-second depending on the weight of the bullet in the cartridge. Additionally, the .223 bullet fires extremely flat–even flatter than the .308 at 200-meters. 

While other platforms exist that shoot .223, the cartridge has become associated with one firearm more than any other: the AR-15. In fact, Eugene Stoner, who developed the revolutionary AR-15 rifle, created the .223 round specifically for his new baby. While AR-15s can be chambered for other calibers–most notably, the 300 Blackout–the .223 is unquestionably the most common AR-15 caliber.

What is .223 Good For?

The US Military’s reasoning behind adopting the .223 over the .308 came down to physics and greater inventory capacity. Because .223 ammo is smaller and lighter than .308 ammo, troops can carry more of it into an engagement while shoring up a greater carrying capacity for other supplies like water, food, or socks.      

A smaller bullet also means less recoil. Specifically, a .223 bullet will generate 4 foot-pounds of recoil. The difference in recoil was one of the major performance improvements that the US Army noticed when it first tested the .223. They discovered that less kick meant it was easier for a shooter to align follow-up shots which, in turn, made an operator more effective at doing their jobs in the field. Additionally, the discovery also made the .223 significantly easier to control when shooting in full-auto.

Why is 223 the Best AR-15 Caliber?

For civilians, one great thing about the .223 cartridge is its availability. Nearly every ammo manufacturer makes some form of .223 ammo these days, including Barnes, Hornady, and Federal.

Russians have even gotten into the .223 game, with Tula and Wolf steel-cased ammo becoming a popular, inexpensive alternative to their brass American counterparts, up until the recent Russian ammo ban, of course.

There’s one more hugely practical advantage to .223: its price. Right now, brass case .223 ammo can be purchased at 42 cents per round online. And steel-cased .223 is even cheaper, coming in at under 33 cents per round. While that cost is significantly more expensive than pre-pandemic prices, .223 is still one of the cheapest AR-15 caliber bullets around.

When compared to the .223, the price of .308 is significantly higher. Brass-cased .308 starting at fifty-two cents per round. Even the relatively-cheaper steel-cased ammo costs 50 cents per round. Yet, .223 is more widely used when firing downrange than .308 is in any case.

One of the only aspects that isn’t ideal about .223 rounds is that some jurisdictions won’t permit hunters to take down big game with it. However, civilians can still use this ammunition-type for getting rid of varmints from their property.

Perfect Mid-Length AR-15 Barrels

One big difference between .223 and .308 is the amount of powder in each cartridge which can have a big impact on the ideal barrel length of a firearm.

When a bullet is fired out of a gun, pressure from igniting powder forces the bullet forward, but powder ignition doesn’t happen immediately. If a bullet leaves the end of a barrel before all of the powder burns off, it loses the rest of the firing-power generated by the powder. This sequence causes the “fireball” effect that emits from the end of some short-barreled firearms.

Generally, a .223 bullet will require about 25 grains of powder to successfully fire a bullet and cycle the action. That portion is significantly lower than the sizable amount needed for a bigger, heavier bullet like a .308, more on that later.

Since there’s less powder in a .223 cartridge, a bullet spends less time in a barrel than it would in cartridges with more powder. This cycle means .223 bullets can fire effectively from barrels that are around 16-inches long which is the minimum legal length for a non-SBR (short-barreled rifle) AR-15.

.308: Huge Bullet, High Power

Where the .223 bullet is small, light, and fast, .308–technically, “308 Winchester”–is the opposite. The .308 is a much slower-moving round, thanks to its larger, less aerodynamic bullet. The muzzle velocity usually maxes out at just under 3,000 feet per second which makes it slower than the .223 right out of the gate. Of course, muzzle velocity isn’t everything, though, as we’ll see later on.

There was a time when the .308 was the US Military’s standard caliber, under the moniker “7.62x51mm” or simply “7.62 NATO.” The Military originally developed the round for its M14 which would replace the 30-06 fired from the venerable M1 Garand. Other military firearms made use of the cartridge, including the FN FAL and the HK G3, but that all changed when the US Army adopted the shiny, new .223 cartridge.

One thing to take note of when building an AR-15 in .308. Technically speaking, building an AR-15 that shoots .308 ammo would be classified and observed as an AR-10. But naming conventions aside, the mechanics of the firearm are essentially the same.

What is .308 Good For?

While the US Military did transition from the .308 to .223, that doesn’t mean .308 is a dead cartridge, far from it. In fact, there are some instances where a .308 can be even more effective than a .223.

One example where .308 ammo surpasses .223 is hunting. In certain states, like Colorado, Illinois, Iowa, Virginia, and Washington, local laws actually prohibit hunters from using .223. As a result, hunters can only legally hunt using a 30-caliber cartridge like the .308.

Legality aside, .308 rounds have some physical advantages over .223. For one, 308 bullets are much bigger and heavier than .223 rounds which means they hit much harder, transferring more kinetic energy into the target. Another result of the heavier .308 bullet is distance–a 308 will usually shoot further than a .223 if shooting conditions are equal. While the max effective range of a .223 is about 500 yards, a .308 round can easily go the distance, reaching 800 yards or more.

What’s more, a .308 will carry more force at a longer distance, too. While a small .223 bullet zipping along can’t transfer 1000 foot-pounds of force past one hundred yards, that’s not a problem for the hulking .308.

Longer Barrels

Remember how we mentioned that a .223 round has less powder than a .308 does? While a .223 only needs about 25 grains of powder, a 308 requires around 45. That’s right, it takes almost double the amount of gunpowder to move a .308 downrange than a .223. That means a longer barrel is needed to successfully use all that powder. While some .308 guns have 16-inch barrels–looking at you, M1A SOCOM–this build isn’t ideal.

Anyone who’s visited an indoor range where someone fres one of these guns, it’s understood that they’re LOUD, and they generate a considerable amount of muzzle flare. The compounded discharge is due to the leftover powder burning off after the bullet leaves the barrel which is not recommended. Rather, for purposes of accuracy and performance, a .308 barrel that’s 20-inch-long or more is advised. Essentially, the .308 is going to be heavier than a gun chambered in .223.

What’s the Best Caliber For Building an AR-15: .223

If you’re looking at the right caliber for building an AR-15, the .223 is the way to go because it’s going to be far cheaper to buy smaller caliber rounds for your weapon. The .223 will also generate less recoil which will make it much easier to shoot, especially if you’re a beginner who’s learning how to aim with accuracy. 

Building an AR-15 can be tricky, but don’t worry, we’re here to help. Check out our selection of AR-15 cleaning and maintenance tools and accessories.

Justin Trump

Justin Trump is the managing editor and owner of CAT Outdoors. The son of a Vietnam veteran, he’s an avid gun enthusiast and 2A advocate. He holds two firearm patents for the CAT M4 and Talon tools. When not managing CAT Outdoors, he enjoys spending time with his family and friends, rooting for Michigan sports teams, and serving his church.