PART 2

SELECTING THE CORRECT BARREL FOR YOUR AR-15 — PART 2: SELECTING THE CORRECT CARTRIDGE

Now that you’re selecting the correct barrel for your next AR-15 build, it’s time to lock in the most critical factor: the cartridge.

Before we go further, let’s make something clear: The caliber is the barrel—it defines the internal diameter of the bore. The cartridge is what that barrel is chambered in. Meaning, barrel makers take a caliber-specific barrel blank and then cut a chamber into it to accept a particular cartridge.

This matters. The cartridge you choose drives everything: your ideal barrel length, gas system (more on that in Part 3), twist rate, recoil characteristics, and how the rifle performs in the role you built it for.

By identifying your primary use case—CQB/home defense/tactical, hunting, competition, or multi-role—you can narrow down the best AR-15 cartridges on the market today. Below, we’ll look at how each shines in its domain.

CQB / Home Defense / Tactical

223/5.56 NATO
Still the standard. Proven, reliable, and supported by a mountain of data and accessories. Works well across multiple barrel lengths, especially 11.5”–16” in this role.

300 Blackout
Designed for suppressed, short-barrel performance. Excellent for home defense or CQB builds in the 7.5”–10.5” range. With modern ammo, it’s more effective than ever—both subsonic and supersonic.

6mm ARC (12.5")
A very capable short DMR or recce configuration. Surprisingly soft-shooting in this barrel length, and with tuning, it becomes one of the most versatile short tactical cartridges available.

22 ARC (16–20")
Similar to 6 ARC in role and performance—offering heavier bullets than 223 with less wind drift and great retained energy. A fantastic option for modern tactical applications.

Hunting

223/5.56 NATO
Often overlooked, but great for small game hunting and varmints with the right projectile.

6mm ARC
Great for predators, hogs, and medium game. Balanced performance and accuracy.

6.5 Grendel
Built for traditional hunting distances. This cartridge hits hard inside 300 yards and is perfect for whitetail and pigs in heavy brush or open fields alike.

22 ARC
A coyote hammer with high speed and excellent terminal ballistics. It’s also very suitable for whitetail and pigs, especially with quality hunting bullets.

300 Blackout
Ideal for short-range shots in thick brush. When paired with subsonics and a suppressor, it becomes extremely quiet and effective on medium game within 100 yards.

Competition / Precision

22 ARC
An emerging favorite. Light recoil, excellent BCs, and a flat trajectory make it an ideal cartridge for gas gun matches.

6mm ARC
Precision with punch. Works beautifully in a tuned 18”–20” DMR platform and has the legs to stay supersonic far beyond traditional AR-15 cartridges.

223 Wylde / 5.56 NATO
Still widely used in action shooting sports like 3-Gun. Great availability and proven performance.

Plinking / Training / Multi-Use

223/5.56 NATO
Affordable, low-recoil, and forgiving—perfect for training and general-purpose builds.

6mm ARC
While more expensive, it’s a highly versatile cartridge and an excellent option for enthusiasts who want consistent performance across multiple roles.

22 ARC
You get flatter shooting, better long-range performance, and minimal recoil. It’s quickly becoming a fan favorite for crossover use.

300 Blackout
While ammo isn’t cheap, it’s a fun cartridge to train with if you already run it for defensive or hunting roles. Subsonics also make it great for low-noise range time.

Quick Note on 223 Chamber Options

There are three popular chamberings for the 223/5.56 cartridge:

• 5.56 NATO — great for tactical shooting; can handle higher pressures.

• 223 Remington — better suited for varmint hunting and lighter projectiles.

• 223 Wylde — a hybrid chamber that allows safe use of 5.56 and 223 Rem while often improving precision. It’s become the most popular chambering for SPRs and DMR builds.

These are all designed for the same cartridge, but differ in chamber geometry and throat dimensions.

Why It Matters

Your choice of cartridge directly influences which barrel is best for you. Cartridge pressure, case size, bullet weight, and typical use case all factor into how long your barrel should be, what twist rate you’ll need, and what gas system will make it run reliably.

For example, building a 6 ARC upper isn’t the same as a 300 BLK truck gun. One needs a tuned gas system and longer dwell time—one thrives in short, suppressed builds. Your cartridge choice guides all of this.

What’s Next?

In Part 3, we’ll break down gas system lengths—why they matter, how to match them to your barrel and cartridge, and what happens when you get it wrong.

It’s one of the most overlooked aspects of AR builds, and we’re going to make it crystal clear. You don’t want to miss this one.