The following was posted on Quarterbore from someone claiming to be R&D for AAC:
If you load your own ammo, pay special attention to all of the normal safety practices. Recommended powder for most supersonic loads is Hodgdon H110.
For subsonic, there are special requirements and nearly all published loads are non-optimal for the 300 BLK upper. Many existing loads were developed for Thompson Center® single-shot pistols, or for ARs by people not aware of the magazine limitations. These loads pay no attention to automatic rifle function and should not be used. Look for a load which results in a cyclic rate of 700 rpm or more. AA1680 is recommended because its bulk density matches the case capacity and it will generate enough gas pressure to cycle the weapon.
Suggested subsonic load:
• R-P 300 AAC BLACKOUT brass
• Remington 7.5 primers
• 11.2 grains of AA 1680 powder
• Case length is 1.368 +0.000 -0.020
• Sierra 220 MK loaded to 2.089 OAL (this length is optimal for reliable feeding from USGI magazines)
• Chamber pressure 21,100 psi.
Existing AR magazines have a rib which normally contacts the 5.56mm case-neck. With 300 BLK ammo, the contact is on the bullet. Because the bullet is a larger diameter, the rib will push the cartridges out of alignment, and can lead to binding potentially resulting in Failures to Feed. For this reason, it is important to load ammunition so that the contact point with the magazine rib is on the bullet ogive in an area of about 0.250 inch diameter. Here are some suggested OAL for popular bullets:
• Hornady 110 V-Max, OAL: 2.000
• 110 TSX, 2.015 OAL
• Sierra 155 Palma, 2.150 OAL
• Sierra 220, 2.089 OAL
• Lapua B416 200 grain, 1.960 OAL
• Hornady 150 FMJ-BT 3037, 2.065 OAL
• Nosler Ballistic Tip 125 grain, OAL: 2.085
• Remington AccuTip 125 grain, OAL: 2.085
• Hornady 130 SP #3020, OAL: 2.010
• Sierra H2120 125 ProHunter, OAL: 1.950
• Speer TNT 125 1986, OAL: 2.010
With a case length of 1.348" to 1.368" it sounds exactly like a 300 Whisper / 300-221 / 300 Fireball which has a trim to length of 1.355". I'm assuming since they recommend AA1680 as the subsonic powder that the system uses a carbine length gas tube instead the pistol length. That is unfortunate as the pistol length has proven to be more versatile when switching back and forth between subsonic and supersonic.
As for the magazine problems caused by the bullet diameter that isn't anything new with AR shooters. I've found that a Pmag with the side rib removed is the best solution as opposed to loading ammo to a restrictive length. This also works well with my 7.62x40 which can't rely on overall length as the case is too long to allow such short loading.
Overall, I don't see anything new here. That said, I think it will be good for the cartridge family to have a SAAMI approved chamber and solid factory backing. I'm still waiting to see if the ammo is interchangable between the Blackout and the Whisper. From what I have read I suspect it may be.