Quoted:
.300BLK is a fun caliber that's even better with a suppressor and subsonic ammo. Imagine shooting without hearing protection and hearing your bullet hit the target 50 yards away, over the sound of the bolt cycling. Most subsonic ammo is suitable for plinking although there are a few rounds out there designed to expand at such low velocities...Get ready to open your wallet for them though. You can find factory subsonic ammo out there for as little as $.50/rd. You can expect velocities around 1000-1050 FPS out of a 16" gun. A bit less out of a short barrel like an 8", 9" or 10.5".

Supersonic ammo is obviously faster and suitable for hunting up to medium game animals such as deer <150yds and obviously for self defense. There's a TON of factory loads out there ranging from about $.50/rd up to $2.00/rd or so. The consensus on one of the most effective round for hunting/defense is the Barnes 110gr TAC-TX but once again, get ready to open your wallet. Velocities will be a bit more "all over the board" with supers but the range is around 1900fps in short barrels to around 23-2400fps in 16"+ barrels...I get around 2100fps with that Barnes load from a 10.5" suppressed barrel.

You can get your cost down quite a bit if you choose to reload and if you're reloading subs especially, they hardly take any powder so your main costs are the projectile and primer, once you have some brass (that you can make out of 5.56 brass if you're ambitious.)

Some have described .300BLK as the "American AK" round for its relatively similar size to a 7.62x39 AK round. (BLK is 7.62x35 and shares similar ballistics).

My opinion, take the plunge. It's a great caliber and is extremely flexible, although more expensive to shoot if you limit yourself to factory loads only. It also only requires a barrel change to make it work in a standard AR. All other parts including the magazines are the same. ( I've had issues with both subs and supers cycling properly in Gen 2 Magpul 20's and Gen 3 Magpul 10's, just FYI).

Quoted:
If you didn't understand the difference between subs and sonic ammo then why are you looking at 300blk? Not trying to be rude just wondering why you're seeking another option as opposed to 556. 556 is a very capable round especially with modern defensive ammo, it is also a NATO round which places the economy's of scale largely in favor of 556 as opposed to other cartridges and makes the ammo more available from more places. So why another caliber?