Reloading Press: .300 AAC

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

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.