Your question perked my interest to I delved a little more into it than what was in my previous answer. Only a barrel change is required to accommodate the difference in the shoulder shape and pressure curve of the 300 blackout. Any 223/5.56 magazine works with this round. As I stated in my previous answer, the 300 blackout is intended to provide greater penetration that what the NATO 5.56.

It's best to build an entire gun in 300 blackout. The tube and buffer spring may not work well in different uppers. One thing to note is that if the bullet is seated deep enough the 300 will chamber in the 223/5.56 chamber and blow the gun up so be careful magazines are kept separate. Color code them if you can.

I started my at collection with a custom built 556 and loved it till I bought a 300 AAC Blackout upper assemble. that lasted about 1 range visit. then I built a lower for the 300BO. Now I'm building a 308 AR.

The transition entails nothing more than to purchase a new upper assembly for your AR-15 or if you're so inclined changing out the barrel. The 300 AAC was designed for the U.S. Military with the requirement that ONLY the barrel need be replacement. So the complete lower, the bolt and bolt carrier group and all your magazines will all still work. Of course, you can buy a complete 300 AAC weapon, just understand that the lower and bolt carrier group are no different functionally than the AR-15 you now have. As to the advantages of having a 30 Cal 125 gr bullet over a 22 cal 62 gr bullet well let's just say the jury is still out - much goes to personal preference. I am sure there are plenty of opinions out there to read. For one: alloutdoor.com/2014/05/22/300-aac-blackout/

The simplest answer is the barrel needs to be changed to accommodate the different bullet taper and higher pressure. The main purpose of the 300 BLK is to provide similar penetration power of an AK47 in anAR-15 type rifle. The velocity of the 300 BLK is about 1/3 less than NATO 5.56/223 but has higher terminal kinetic energy at a given distance. I do know that the AR-15 magazines work’s with the 300 BLK. However, I’m not sure that the buffer spring is adequate and may also require gas tube pressure adjustments as well. I understand that kits are available and that might be the best was to go.

As noted previously, the transformation is not difficult. As to the advantage, some would cite the biggest advantage being the relatively lower velocities associated with the 300blk out. It is suited for a suppressor, which is an important consideration for some. For hunting, it also has a different level of lethality (maybe better/maybe worse) than the 5.56, depending on many factors including game, bullet weight/construction, distance, shot placement, etc. I'd never argue against getting another gun that you're interested in... and in this case it's sort of like getting another 1/2 of gun, so the decision is that much easier.

Going from 5.56 to .300 blackout is just a barrel swap. if you have an armorer's wrench and a bench block you can switch between the two with only a bit of effort. As for uses, it's all situational. 5.56 has a flat trajectory, good penetration, and is generally pretty cheap. .300 blackout is basically 7.62x39 that fits in a STANAG magazine and works really well as either a full power rifle round or as a heavy subsonic round for shooting suppressed. Personally I would recommend just getting something in 5.56 to start because the ammo is cheaper and it still hits hard enough to stop anything person/deer sized or smaller.

Its a very simple conversion (basically just changing your upper). But unless you're hunting big game and can't aim well enough to kill it with a 5.56, its not worth it. The 300 is more umph knockdown force. The 5.56 rips right through you. I like the penetrating power of the 5.56 better. The 300 blackout is just a new thing that seems hot right now. But if you do it and decide its not for you, just switch back. Just don't mix you're ammo up, that can be a deadly mistake.