.300 AAC Blackout is a niche cartridge. It will never rival the general versatility of a mainstay round like .223 Remington/5.56 NATO or .308 Winchester, but for its intended purpose, it performs better than almost any other round in its class.
As a specialty round, though, it requires a different set of considerations and characteristics in order to optimize its advantages. It’s well known that converting a standard AR-15 from 5.56x45mm to .300 Blackout requires only a barrel swap, but below, we’ll take a look at how barrel length impacts .300 Blackout performance, and how that differs from the effect of barrel length on 5.56.
Understanding the .300 Blackout Cartridge
While rounds like 5.56 have been around for ages and achieved a certain level of ubiquity, more specialized rounds are less common, leaving many wondering, “What is .300 Blackout?” .300 Blackout is a specialized round developed for a specific purpose, and that purpose is suppression. Even the name draws attention to its priority of stealth.
The round was first developed as a way to improve the suppressed performance of the M4 carbine and similar rifles. While M4s are easily tuned to run perfectly with suppressors, the 5.56 NATO cartridge left much to be desired.
That cartridge relied on velocity for its terminal effectiveness, meaning that supersonic ammo was required to maintain efficacy, which limited the degree to which the round could be quieted. Using subsonic 5.56 ammo would result in a ballistic profile not unlike that of a .22LR, which, for obvious reasons, was undesirable. A full breakdown of the differences between .300 Blackout vs 5.56 can be found in our guide on the subject.
The solution was to develop a round that could utilize a larger, heavier bullet that could still deliver sufficient energy to the target even at subsonic velocities, allowing for both effective suppression and effective fire.
This led AAC to introduce the .300 Blackout in 2010. From there, it rapidly grew in popularity with police, military, and civilian users alike. It didn’t hurt that the conversion from 5.56 to .300 Blackout required only a barrel swap, and the ballistics of supersonic loads were reminiscent of that of 7.62x39mm, a round well-regarded for its short- to mid-range performance. A full breakdown of the pros and cons of each round is available in our article on .300 Blackout vs 7.62x39mm.
In addition to its exceptional suppression characteristics, .300 Blackout came with a secondary benefit: short-barrel performance. Because the round utilizes a relatively heavy bullet and fast-burning powders, most loads see full powder burn relatively quickly, with incremental velocity increases dropping off sharply after 8-9″ of barrel length.
This meant that unlike 5.56, which is often regarded as having a minimum effective barrel length of around 10.3″, .300 Blackout could utilize much shorter barrels while still maintaining sufficient ballistics and terminal energy. This further boosted its utility as a suppressed round, as a shorter barrel is a high priority for a .300 Blackout rifle that will utilize a suppressor full-time.
Factors to Consider When Choosing Barrel Length
There are several factors to consider when choosing the length of your .300 Blackout barrel. Some are specific to your use case, while others are true for nearly any application of the round. When building or buying a new rifle or .300 Blackout upper, it’s a good idea to carefully consider each of these before making your selection.
Ballistics
The first thing to understand is how barrel length will impact your ballistics. With nearly any cartridge a longer barrel will mean more velocity—up to a point. More velocity generally means a flatter trajectory and less wind drift, which will make hits at extended ranges easier.
However, it’s also true that each extra inch of barrel length comes with diminishing returns. As such, the jump from an 8″ barrel to a 9″ one will be more significant than that from a 20″ barrel to a 21″. This means that for each cartridge there is a sweet spot or range in barrel length, after which it is generally not worth going longer unless your use case necessitates maximum velocity and is not affected by additional size or weight.
For .300 Blackout, the optimal range is often considered to be around 8-9″. At this length, most .300 Blackout loads will achieve full powder burn and produce sufficient velocity to adequately perform at the ranges .300 Blackout is generally used for.
While a longer barrel would yield more velocity, the difference is not typically going to significantly impact performance—no matter how long your barrel is, you’re never going to turn .300 Blackout into a long-range precision round.
While shorter barrels are viable and are sometimes used, they give up a significant amount of velocity, thereby limiting the range and efficacy of the round. So, while guns like the Sig Rattler with its 5.5″ barrel do exist and have their place, a barrel shorter than 8″ is going to be ballistically sub-optimal for .300 Blackout.
Only firearms being deliberately optimized for minimal size and maximum maneuverability in confined spaces should consider barrels shorter than 8″.
Overall Length
Depending on your use case, the overall length of your firearm will range from paramount importance to minimal. .300 Blackout tends to be a relatively short-range cartridge, often selected for home defense or close-quarters roles where overall length is very important. However, it’s also sometimes used for hunting—a role in which overall length typically has no impact on performance.
One key factor will be whether or not you intend to use your rifle with a suppressor. If you do, consider that even a highly compact suppressor will typically add 3-4″ to your overall length, with larger models adding up to 8″. As such, if overall length is at all a priority, you’ll want to keep your barrel as short as possible without handicapping your ballistics.
This again brings us back to an optimal range of 8-9″, although, for truly short-range-dedicated firearms, a shorter barrel length may be used to limit overall length to the absolute minimum.
NFA Status
As you may have noticed, for most applications, an 8-9″ barrel is generally recommended. However, there is one primary drawback to such lengths, and that is their legal restrictions.
Opting for a barrel shorter than 16″ requires you to either build or buy your firearm as a pistol, limiting your options for accessories and attachments, or file for a tax stamp and build an SBR.
If your use case or preferences require accessories that are not compatible with a pistol classification and you prefer not to SBR your firearm, whether due to state legal restrictions or preference, your only remaining option is a 16″ or longer barrel.
The Best Barrel Lengths for .300 Blackout
There are a wide range of different .300 Blackout barrel lengths, each suited to a different purpose, but the vast majority of firearms are best served by one of three barrel lengths.
8-9″
The king of all-purpose .300 Blackout firearms, the 8-9″ barrel length is to .300 Blackout what the 14.5-16″ length is to 5.56 guns. It’s the perfect sweet spot of the velocity-to-length ratio that lets you take full advantage of .300 Blackout’s ballistic capabilities while minimizing size and length, saving room in both attributes for the inevitable suppressor.
You’ll find a huge variety of options in this range, both for prebuilt firearms and individual barrels.
5.5-6.75″
.300 Blackout is often chosen for its short barrel performance and short-range efficacy. For a dedicated home defense gun where performance at longer distances is irrelevant and maneuverability around corners and through doorways is paramount, a 5.5-6.75″ barrel offers minimal length while still maintaining reliable function.
These micro-length barrels give up some ballistic capability, but for applications where the tradeoff is acceptable, they deliver the most compact package possible.
16″
While the increase in velocity is typically not considered to be worth the increased size and weight, there are a few situations in which a 16″ barrel makes perfect sense.
Chief among them is hunting; you won’t find many tight corners or confined spaces in the forests and fields. If your .300 Blackout is a bolt-action rifle or even a hunting-focused AR-15, a 16″ barrel will maximize your velocity and ensure minimal muzzle flash even with the slowest burning powders.
16″ barrels are also an excellent choice for those for whom pistols and SBRs are not an option.
Conclusion
.300 Blackout is a specialty round and requires a specialized barrel to make the most of it. For typically .300 Blackout use cases, an 8-9″ barrel is often the best balance, but shorter barrels may also be used in situations that call for them. And of course, there will always be a place for a 16″ barrel, whether for legal restrictions or hunting purposes.
