When you’re building or buying an AR pistol (ARP) or short-barreled rifle (SBR), barrel length matters more than you might think. The difference between a 10.5-inch and a 16-inch barrel affects everything from velocity and terminal performance to how the gun handles and sounds when suppressed. And if you’re choosing between 5.56 and .300 Blackout (aka .300 AAC Blackout or 300 AAC), those choices multiply.
To help you choose the right factory ARP/SBR or components for your future build, we’ve prepared this guide that explains how different barrel lengths impact the gun’s performance, efficiency, and reliability, for both 5.56 NATO and .300 Blackout.
Under the National Firearms Act (NFA), a rifle must have a barrel of at least 16 inches. Anything shorter requires you to register it as an SBR, which involves a tax stamp and some paperwork. The alternative is buying/building an “AR pistol,” which uses a stabilizing brace instead of a stock, sidestepping the SBR classification.
5.56 vs. 300 Blackout
Before we talk about barrel length, it’s good to remember what each cartridge was designed for in the first place. The 5.56 NATO round was created to be effective out of a 20″ barrel, relying on high velocity for its terminal performance. When you start chopping the barrel down, you lose that velocity, and with it, some of the round’s effectiveness.
The .300 Blackout, on the other hand, was built from the ground up for short barrels. There are many sources that claim that .300 BLK is “optimized for 9-inch barrels” or that “beyond 9 inches, you don’t gain anything.” This is not true. While .300 BLK can be effective in a 9-inch barrel, it’s not optimized for that length, and longer barrels do provide benefits.
Overview of Barrel Lengths for 5.56 NATO AR Pistols and SBRs
7.5″ barrels represent the extreme short end. You’ll see velocities around 2,400 fps with 62gr ammo, but reliability becomes questionable – short dwell time often causes cycling issues, and the massive muzzle blast creates serious concussion problems.
10.3″ is the length of the military’s Mk18 and a popular choice for clone builds (you may also see guns with a 10.5″ barrel). The extra length improves reliability substantially compared to shorter barrels while keeping the setup compact. Velocity increases to approximately 2,450-2,600 fps. Still, these barrels are notoriously loud and produce a massive fireball. While better than in 7″ barrels, the dwell time is still short, making 10″ AR pistols/SBRs somewhat finicky.
Military requirements often drive civilian preferences, but these decisions aren’t always based on optimal performance. The military didn’t choose a 10.3″ barrel for the Mk18 because it was optimal – rather it was the shortest configuration that could still cycle reliably. For CQB in vehicles and tight spaces, every fraction of an inch mattered more than ballistics.
11.5″ barrel configuration proves more forgiving than 10.3″, offering the sweet spot for 5.56 pistol builds. That additional 1.2 inches of barrel provides meaningful velocity gains due to better dwell time without losing much maneuverability. While 150-200 fps might seem insignificant on paper, it becomes crucial when using ammo designed for specific velocity thresholds. Generally, the system becomes much more forgiving with ammo selection. Particularly, 77-grain open-tip match bullets rely on minimum velocities around 2,300 fps for proper terminal performance.
12.5″ barrels allow mid-length gas systems, which dramatically smooths recoil impulse. This setup is still compact, but you’ve gained enough velocity to stretch your effective range past 200 yards.
13.7″ and 13.9″ barrels exist mainly for one reason: they allow you to reach the legal 16″ minimum with a pinned and welded muzzle device, avoiding the SBR tax stamp. It’s a workaround, but it locks you into that muzzle device unless you want to pay a gunsmith to redo the work. The same goes to the 14.5″ barrel, a standard for the M4 carbine.
11.5 inches of barrel length is considered by many the minimum for a serious 5.56 SBR, especially if you’re running a suppressor.
DIAMONDBACK DB15 5.56mm Pistol
$661.99
Price accurate at time of writing
- Caliber: 5.56 NATO
- Gas system: Pistol Length
- Barrel length: 7"
- Barrel twist: 1:8″
- Capacity: 30rd
Overview of Barrel Lengths for 300 BLK AR Pistols and SBRs
If choosing a 5.56 pistol means compromises in performance, 300 BLK was engineered to perform well in short barrels specifically, achieving nearly complete powder burn in a 9-inch barrel. Its real beauty is in its versatility, though – you can switch from powerful, hard-hitting supersonic rounds to quiet subsonic rounds with just a magazine swap.
7″ to 10.3″ are ultra-short barrels that deliver the 300 BLK’s intended performance. Subsonic 220-grain loads maintain around 950-1,000 fps, perfect for suppressed use. Supersonic loads still generate nice velocities – 2,100+ fps with 110-grain bullets from a 7.5″ barrel. The trade-off comes with increased muzzle blast and concussion when shooting unsuppressed. If you’re not planning to run a can, consider going longer.
The 12.5″ barrel offers excellent balance between compactness and performance. It can push supersonic 110-grain loads to 2,200 fps and 125-grain loads to around 2,100 fps. For hunting, this barrel provides enough velocity to reliably expand hunting bullets at longer ranges.
A 16″ barrel can push 110-125 grain supersonic rounds to 2,200-2,400 fps – nearly matching some .30-30 Winchester loads, effectively taking medium-sized game at distances up to 200 yards.
Keep in mind that some states have minimum barrel length requirements for hunting rifles, making a 16-inch barrel a necessity.
DANIEL DEFENSE DDM4 PDW .300 Blackout AR-15 Pistol
$2,279.00
Price accurate at time of writing
- Caliber: .300 AAC Blackout
- Gas system: Pistol Length
- Barrel length: 7"
- Barrel twist: 1:7″
- Capacity: 30rd
Choosing the Right AR Pistol / SBR Barrel Length for Your Application
Home Defense
Both calibers work, but they have different strengths.
If you live in an apartment, 5.56 might be the better choice. It loses energy faster after passing through drywall, reducing the risk to neighbors. A 10.5-inch or 11.5-inch barrel keeps the gun compact, and if you’re willing to deal with the noise, it’s effective.
If you want to run a suppressor and you have the space (or live in a house), a .300 BLK pistol makes more sense. A 9-inch barrel with subsonic 300 BLK ammo gives you hearing-safe shooting indoors, which protects your family’s ears. Just be aware that .300 Blackout penetrates barriers more effectively, so you need to know what’s behind your target.
Hunting
For small game and predators, 5.56 works fine out to 100 yards. Use a 16-inch barrel for best results.
For medium game like deer or hogs, .300 Blackout is the better pick. A 12.5-inch or 16-inch barrel with 110- to 125-grain supersonic rounds delivers enough energy for ethical kills inside 200 yards. The heavier bullets penetrate better and do more damage than 5.56.
If you’re hunting anything larger, or shooting past 200 yards, you need a bigger cartridge.
General Purpose
If you want one gun that does everything reasonably well, go with a 10.5-inch to 11.5-inch barrel in .300 Blackout. You’ll have a compact package that works suppressed, handles home defense, and can hunt medium game at moderate ranges.
Alternatively, buy a 16-inch 5.56 rifle and a separate .300 Blackout upper. Swapping uppers on an AR-15 takes seconds, and you’ll have the best of both worlds.
A Word on Safety
If you own both calibers, you need to be extremely careful. A .300 Blackout round will chamber in a 5.56 barrel. If you pull the trigger, the barrel will explode. This isn’t theoretical – it happens, and people get hurt.
A Few Things to Consider
Suppressor Plans
If you cannot own a suppressor, the .300 Blackout loses one of its primary advantages. The 5.56 becomes more competitive in this scenario, though .300 Blackout still maintains an edge in reduced concussion.
Over-Penetration
Modern defensive ammunition in both calibers performs well. Properly selected 5.56 rounds and .300 Blackout V-Max rounds both offer controlled penetration suitable for home defense.
Gas Systems and Suppressor Tuning
.300 Blackout runs much cleaner suppressed than 5.56. Most setups work fine with a standard carbine buffer, though you might want an H2 or H3 buffer if you’re getting too much gas in your face.
Both pistol-length and carbine-length gas systems work for .300 Blackout. Pistol-length systems are common on shorter barrels, while carbine-length systems show up on 12.5-inch and longer barrels. As long as the gas port diameter matches the system length, you’re good.
For 5.56, mid-length gas systems on 12.5-inch to 16-inch barrels offer smoother recoil and less gas blowback than carbine-length systems. If you’re building a suppressed 5.56, an adjustable gas block helps manage overgassing.
Budget
Consider the total cost of ownership. While a quality rifle in either caliber represents a significant investment, ongoing ammo costs favor 5.56.
5.56 ammunition costs about half as much as .300 Blackout. If you shoot more than 1,000 rounds a year, that adds up. But if you only shoot a few hundred rounds annually, the cost difference might not matter.
.300 Blackout is also a popular reloading caliber. If you handload, you can cut your costs significantly and tailor your ammo to your specific barrel length.
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