8.6 Blackout: A Subsonic Assault Cartridge

8.6 Blackout: A Modern Subsonic SMG Assault Cartridge

We’ve spent a lot of time highlighting 8.6 Blackout in it’s capacity as a flexible hunting cartridge for short-barrel systems, around 12 inches. In fact, a few members of the Phantom Team will be taking to the mountains of Utah, Colorado, and Montana in pursuit of elk this fall with different bolt platforms chambered in 8.6 BLK to test new products. In this article however, we intend to focus on 8.6 BLK in it’s potential as a dedicated subsonic “submachine gun” assault cartridge. 8.6 Blackout is increasingly becoming a subject of serious evaluation among end users. The combination of compact form factors, heavy subsonic payloads, and potent supersonic capability positions 8.6 BLK as a highly flexible solution for modern, high-speed assault environments. As mission demands evolve, so do the requirements for weapons and ammunition that can serve across a wide range of roles with minimal compromise. 8.6 Blackout appears poised to meet that demand in full and serves to complement existing weapon systems well.

Operational Flexibility in a Compact Package

When configured in 6"–8" barrel platforms, 8.6 Blackout can deliver an impressive balance of maneuverability, suppressibility, and performance. There are plenty of these systems currently being tested and developed, including the Boombox by Q along with several other concepts. These systems are compact enough for rapid movement through confined environments—vehicles, urban structures, aircraft—while retaining meaningful ballistic capability for extended-range engagements beyond what is typically expected from weapons of this size. Subsonic loads with projectiles in the 400+ grain class provide exceptional terminal performance, producing up to 1,000 ft-lbs of kinetic energy at the muzzle. While kinetic energy is not the sole metric of terminal effect, it is still a useful indicator of the round's power. For comparison, 220 gr 300 Blackout subsonic rounds generally deliver closer to 500 ft-lbs of kinetic energy. Another interesting comparison would be that 77 gr 5.56 is producing very nearly the same muzzle energy (approx 1090-1100 ft-lbs) in an 11.5" barrel. That comparison is not super meaningful other than to provide a benchmark for what door kickers may already be working with in terms of kinetic energy at the muzzle.

These long, heavy rounds provide barrier-blind performance in many cases, making them ideal for assault scenarios where obstacles are present. When a mission demands increased energy delivery or longer effective range, supersonic loads—with 200-210 grain projectiles—are capable of reaching north of 2,000 feet per second from an 8" barrel, based on prototype Phantom loads using the 8.6 Blackout cases we have developed alongside SST using NAS3 case technology. The ability to switch between these profiles without modifying the weapon platform allows the operator to respond dynamically to changing conditions in real-time. These supersonic rounds would be capable of delivering over 1700 ft-lbs of kinetic energy at the muzzle. This is just shy of 168 gr 308 traveling at 2250 fps from a 12.5" barrel, which would produce almost 200 ft-lbs more kinetic energy, but would require 4.5" more barrel to do so. Again, nothing is really meant by that comparison other than to establish a benchmark. We aren't suggesting that this package necessarily has more extended range capability than 308—that’s a whole different discussion. In fact, we'd argue that 308, 6.5 CM, and 6 CM are all necessary complements to the subsonic submachine gun assault platforms we are highlighting.

A Growing Area of Evaluation

Across multiple communities, from special operations units to R\&D-focused military programs, there is active evaluation of 8.6 Blackout and its potential role in near-peer and asymmetric conflict environments. The core advantage being assessed is flexibility: the capacity to conduct suppressed, low-visibility engagements with devastating subsonic ammunition, and the immediate ability to escalate to supersonic fire when required—all without sacrificing mobility or increasing weapon footprint.

This dual-role capability simplifies logistical demands, reduces the number of platforms needed for mission-specific configurations, and creates greater continuity in training and system familiarity. As interest continues to build, more data is being gathered in field trials, shoot-house testing, and controlled evaluation environments. The trend line is clear: end users are seeking more from their compact weapons, and 8.6 Blackout is capable of delivering more.

Shorter Barrels are More Accurate with Subsonic Ammo... Maybe?

One theory gaining traction among developers and shooters is that shorter barrels may actually enhance accuracy when using subsonic ammunition. This is literally a theory based on eliminating the impact people have on the gun. Because subsonic projectiles spend more time in the barrel due to their slower velocity, they are exposed longer to any disruptions caused by shooter movement or inconsistency. A shorter barrel, by definition, shortens this window, potentially reducing the likelihood of shooter-induced error at the moment the projectile exits the bore. Again, we want to emphasize that is a THEORY. We have not proofed this out but it does make for some cool thought experiments.

In this light, the 6"–8" barrel length range not only supports a highly compact and maneuverable platform, but also may yield more consistent subsonic accuracy. When paired with stable flight characteristics of long, heavy bullets, the result is a package that can be capable of balancing precision and power in ways few systems can match.

Modern SMG Assault Weapons (SMGAW)

The systems themselves (at least the ones we have tested) are very rugged and reliable. There is a lot of capability that is being packed into these small systems. Almost all of these systems we have evaluated have utilized a 1:3 twist rate which is commonly associated with the 8.6 BLK cartridge. There are some 1:5 twist platforms we have seen as well though, we will leave it to the end user to determine which platforms are preferred but at this point the Phantom team is seeing more impressive performance with the faster twist platforms, for the most part. 8.6 Blackout as a cartridge itself challenges the traditional definitions of a submachine gun cartridge. In its subsonic configuration, it brings very near rifle-level kinetic energy, greater environmental resilience, and a payload that retains lethal effect even after penetrating intermediate barriers, while maintaining very high levels of suppression capability. Projectile selection is diverse, whether a subsonic expander is selected or a terminally performing fracturing bullet is preferred, both are available and could be utilized for different purposes and use cases. Either of these projectiles outperforms subsonic 300 blackout in terminal capability. Unlike pistol-caliber subguns, 8.6 offers predictable, consistent terminal performance across a broader range of engagement distances as well as higher levels of accuracy and precision.

With supersonic ammunition, 8.6 BLK breaks through the limitations normally associated with short-barrel rifles. Phantom supersonic loads with NAS3 cases produce impressive external ballistics in these sbr platforms, making it feasible to use a single weapon system for tasks that would traditionally require switching to a full-length rifle. This versatility means fewer compromises and more capability per ounce and inch. These supersonic ammunition offerings are still being evaluated, internally and externally and we will hopefully be able to share more information and details around them soon.

We see modern SMG Assault Weapons being built in the form of small-frame AR-10's. These are essentially AR-15-sized receivers designed to handle cartridges like 308, 6.5 Creedmoor, 6 Creedmoor, and of course, 8.6 Blackout. This commonality opens up additional operational benefits: end users can simply swap uppers to run a longer barrel or shift to a cartridge that excels at extended distances beyond the designed limits of 8.6 Blackout. Does this signal the end of 5.56 or 300 Blackout? Probably not. The AR-15, AR-18, and their many variants have proven staying power and will likely remain in widespread use. However, the military’s interest in Sig’s XM7 and the 277 Fury cartridge shows a clear willingness to reexamine the battle rifle concept—and it’s worth noting there are arguably better ways to approach the problems that system was supposed to solve. By utilizing Shellshock’s NAS3 case technology Phantom can match and exceed the intended performance of 277 Fury with cartridges like 6 Creedmoor, 6.5 Creedmoor, and even .308—without the need to operate at 80,000 psi. And on top of that, users still retain the ability to run 8.6 Blackout from the same lower. While this may stray from the immediate topic of SMG-assault platforms, it highlights the broader ecosystem benefits that 8.6 Blackout brings to the table when implemented within a flexible and modern small-frame battle rifle system. The vision of this ecosystem is why we are taking meaningful steps to build out performance ammunition across the above mentioned cartridges, starting with 8.6 Blackout.

8.6 Blackout serves to redefine personal offensive weapon engagement capabilities. In high-speed assault environments—urban, rural, or maritime—where adaptability, precision, and raw stopping power all intersect, 8.6 Blackout stands out. The cartridge has matured. Now the focus is on implementation. Modern combat packages are increasingly about fitting large-frame performance into small-frame guns. We see tremendous promise in 8.6 Blackout as a modern subsonic sub gun assault cartridge, and believe it will complement 6.5 Creedmoor, .308, 6 Creedmoor (and a few other large rifle cartridges) extremely well—offering impressive capability across the entire battlefield.