300 AAC Blackout

300 Blackout - For Noobs

Since this is a brand new cartridge here, here are some initial thoughts about it not thinking about it or researching it yet.

Its more tricky to get a gun to run "correctly" with. Supersonic velocity is just like any other AR15, but if you want subsonic, its not straight forward really. Usually, you read a load chart from some manufacturer and it says min-max, etc and its all supersonic, and the min and the max will cycle the gun easily. Typically. 300 Blackout, the subsonic load data does NOT typically cycle the gun "easily" until you have a silencer on it that has good back pressure. Of course, powders like CFEBLK do cycle the gun at subsonic WITHOUT silencer, and it does it quite well, but I found 1680 and N110 do not easily. "easily" is key word. This is with standard BCG (Bolt Carrier Group) weight, and standard 3.0 oz buffer and standard buffer spring. If you go with lightweight BCG, or Piston system or -20% power buffer spring, or all in combination, it will allow the gun to cycle with loads that previously would not (under full gas). BUT, the downside is that lightweight system is nowhere near as reliable as the standard equipment. Especially the lighter buffer spring. So try and get it to run using standard weight components using CFEBLK powder to start. Best bet.
The shorter the barrel the easier it is to get gun to cycle, because you can load slightly more powder for the same velocity. So I am using a barrel near 10". Its slower than if I used 16" Barrel, not a ton slower, but slower. So I can use more powder to get me same velocity subsonic. This helps cycle.
The bullets have hugely variable load length (OAL) than standard 5.56/223. While 223/556 are almost without exception from around 2.180-2.260 I am finding 300 BLK is like 2.040-2.260. That is GIANT range.
300 BLK have "slightly" different magazines than 223/556 for AR15. very subtle, but its different. However, that being said, shooting 556 PMAGS I had 0 issues. Not to say its ideal, but they work.
All you have to do to run 300 BLK in your AR15, literally, and I mean "exactly" is change the barrel/muzzle device. So get 300 BLK barrel, then a .308 5/8" x 24 Pitch muzzle device. Standard AR15 take 1/2"x 28 pitch and are smaller diameter.
Components for 300 BLK are actually quite similar price to 223/556. I would say you will pay slightly more for brass and bullets, but its not that much more. Powder is cheaper, because you are using like ~10-15 grains of powder compared to 223 where you are typically around 24-28 grains.
300 BLK is FAR FAR FAR FAR FAR more safe to reload than 223/556. To get near 1100 or less FPS using the 4 powders I shown in pic above, the pressure is very low, nowhere near max chamber pressure. For example, in Hodgdon load data, you can load around 12 grains of CFE Black for a 190 grain bullet, and they say pressure is under 20k PSI. But the 180 grain bullet load is 19.3 grains at only 51k PSI. The moral of the story is: you will be at compressed loads crushing your ogive of the bullet trying to seat it, and you still will be acceptable pressure the heavier bullets. Versus 5.56 if I put in say, 1 grain over max in some bullets, it will get super hot, super fast. Not the case with heavy subsonic 300 BLK. In this example, that is going up approx ~+7 grains of powder, which is over 50% more, and it will be near SAAMI pressure. But those heavy bullets are long, so you will compress big time. You are basically dealing with a Rifle Cartridge that loads to like 45ACP pressures (less than 20k PSI or so) using nearly the same bullet weight (190-220). So it has energy like a 45 ACP, etc.
I noticed, even subsonic, the recoil of 300 BLK is more than 5.56. Considerably more recoil. This is partially due to 300 BLK load-outs are typically much lighter guns than AR15 5.56 due to short barrel, no shouldered butt-stock, and very short and light hand guards.

So that is some basic information from first day on the range with it, trying a bunch of stuff to learn about it.