The Reloading Press is an at-least-weekly feature here on Gaming Ballistic for 2016. Each week it looks at some interesting real-world cartridges and presents them with hopefully-useful information in GURPS Format.

7.62x35mm AAC Blackout


The 7.62x35mm AAC Blackout was developed to create a .30 caliber cartridge that could be fired reliably from rifles with a particular set of design constraints, mostly owing to the desire to use standard M4/M16 magazines and lower receivers, and to retain the 30-round capacity of standard magazines. Having a weapon that would autofire reliably (the 7.62x39mm does not in M16-shaped magazines) is generally a good thing, and having all that in combination with a more-lethal, more-quiet subsonic projectile was a design goal.

Given the popularity and number of variations that seem to be springing up for this near-wildcat, it would appear to have been somewhat successful. The original cartridge is supposed to have been designed around a military request for short-barreled rifles. The manufacturer took the design public. 

The odds of this replacing the 5.56x45mm as a primary cartridge are probably about zero. Even if the military moved away from the M855 family of rounds, I have to think they’d go for something closer to the 6.8x43mm rather than the .300 BLK. But that’s strictly opinion.


Ballistic’s Calculator Inputs

Basic inputs for the calculator are as follows, selecting 406mm for the (test) barrel length. The difference in mission for the subsonic round means that it can’t really be used for checking consistency, but the lighter-weight bullets (at basically equivalent energy) can.

INPUT – Supersonic
.300 AAC Blackout
Chamber Pressure 53500 psi
Barrel bore 7.62 mm
Case Length 35 mm
Chamber Bore 9.2 mm
Barrel length 406 mm
Bullet Mass 125 grains
Aspect Ratio 3.7 L/Bore
Burn length 10.83 mm
Projectile Caliber 7.62 mm
Total Accelerated Mass 125 grains
Expansion Ratio 2 expansion
Projectile Load 1

The subsonic bullet is very different in both design and performance. It requires different inputs, being less than half the pressure of the supersonic round.

INPUT – Subsonic
.300 AAC Blackout
Chamber Pressure 21000 psi
Barrel bore 7.62 mm
Case Length 35 mm
Chamber Bore 9.2 mm
Barrel length 406 mm
Bullet Mass 220 grains
Aspect Ratio 5.1 L/Bore
Burn length 7.57 mm
Projectile Caliber 7.62 mm
Total Accelerated Mass 220 grains
Expansion Ratio 2 expansion
Projectile Load 1

Output Stats

Again, we’ll need two different charts for each weapon. The subsonic one is (to me) the more interesting, since it actually has a different job than the longer range rounds. I’ll give charts for both.

Supersonic 125gr projectile

The muzzle energy for this load with 406mm of barrel is 1,486 Joules

Subsonic 220gr projectile

The speed of sound is just over 340m/s (which is 1,115 fps). 

Notes
  • The velocity is at the muzzle. The velocity with both cartridges is tuned to the 16″ test barrel.
  • The 125gr bullet is just large enough to make a pi –> pi- transition not relevant. Instead, the real question, given the size of both projectiles, is if it gets a pi+ rating at very short range. Canonically, the answer to this is “no,” but at less than 100 yds from a reasonably-sized barrel, it might be quite nasty. If you like, and want to see players flock to this one, give it pi+ below the range listed, and pi otherwise.
  • No one is going to use this round in a barrel much more than 20″ long (maybe not even that), but the model is within about 60fps for a 78gr projectile from a 610mm barrel . . . so for what it’s worth, it’s accurate enough.
  • The point at which barrel friction slows the projectile more than powder gas pushes it is likely well before 607mm for the subsonic projectile. I suspect a 16″ barrel for 3d is about ideal.
  • Rumor has it that the .300 AAC Blackout at full power will still cycle from a 4.5″ barrel, which is why the table stops there.
Most of the alternate projectiles seem to have roughly the same energy and thus will do more or less the same damage from the test barrel. There’s enough mass in the bullet that getting a JSP to expand to a factor of 2 shouldn’t be a problem. While high-mass, short pistol cartridges tend to convert in the model using a rule that is mostly -1 penetration per die of damage, rifle-shaped fast-cartridges retain penetration better, and are closer to -1 per 2d. That would be about 5d-2.5, call it 5d-3, or 14.5pts. Calling it 4d pi+ is about right. 

The huge aspect ratio and high mass of the 220gr subsonic projectile makes my calculator want to give it 3d pi+ even for the regular FMJ round, and if it expands or tumbles, really wants to give it 2d+1 pi++ for a JHP. That might explain why one keeps hearing that for close-in work with suppressed rifles, certain groups of people who don’t exist definitely don’t want to use this cartridge. Which probably also doesn’t exist. 

In any case, if you’re feeling generous, 2d+1 pi++ for the subsonic JHP or JSP – still -1 per 2d, and bump it up one step from pi to pi+ is good for .300 AAC.

Platforms


There are basically two kinds of platforms for this chambering: AR15-style platforms, and bolt-action tactical or hunting rifles. There are many, many AR-15 style uppers that can be mated to any AR15, M16, or M4 lower – which is the part that the US Government considers the actual gun. 

The venerable and high-quality Remington 700 can be had in .300 AAC Blackout, and the AAC Model 7 can be tricked out with a suppressor. Given the heavy bullet and long 1/2D range, making a subsonic, suppressed bolt-action rifle for the precision delivery role might not be entirely insane. 

The last and most populous category will be a plethora of AR-15 (or M4/M16, as you prefer) lower receivers mounted with a suitable upper. Some of the suppressors that are built to go with this cartridge are very, very impressive, with the AAC SR-7 managing 39dB of noise reduction.

They don’t seem to come cheap. Most are $2,000 or $3,000 for a complete rifle, integrally suppressed or not, and most closer to $3,000 than not.

That being said, going straight to the source a 9″ barrel upper receiver ($1,000) plus the 39 dB suppressor (also about $1,000) will probably deliver 2d pi++ (220gr JHP) from the equivalent of a 16-17″ barrel length (9″ for rifle, 7-8″ suppressor) with impressive (lack of) acoustics.

5 thoughts on “Reloading Press: .300 AAC

  1. JD Jones of SSK should prolly be mentioned in the development of this round. Especially since lots of .300 Whisper guns can shoot .300 AAC Blackout just fine.

    Then the hobbyists who wildcatted the identical .300/.221 Fireball.

    This round was around for a LONG time avoiding paying the extortionate licensing fees SSK wanted.

    AAC being backed by Remington is how it finally got market acceptance.

  2. All sales literature for the .300 Blackout indicates that the round is specifically designed to ‘burn out’ in a 9″ barrel, at least in the subsonic loading. I know that this does not mean that a longer barrel would not add *any* velocity, but chronographs seem to show a very limited improvement after about 10.5″ barrel.

    See, for example, this: http://www.vuurwapenblog.com/reviews/tests/300-aac-blackout-subsonic-velocity-data/
    (you may get problems loading the pictures, which include the graphs, but some of the vital data is repeated in text)

    I’ve also seen .300 Blackout loads where only 8% velocity is lost by going from a 16″ barrel to somewhere around a 10″ barrel.

    Could it be that tuning the velocity to a 16″ test barrel is not representative for the subsonic performance, designed for much shorter barrels?

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