Hello, new guy here so go easy on me.  I am working on my first AR build at the moment and I'm getting close to having to pick my caliber.
.223/556 is an obvious choice for reasons of commonality, availability, etc etc.
However, 6.8, 6.5, and specifically .300 interest me a lot.
I will mainly be shooting paper/steel depending on which caliber I end up picking it might see some use as a deer rifle.

6.8 and 6.5 have almost zero availability in my area while .300 is everywhere and at reasonable prices.
I have no interest in shooting subsonic through a silencer, but I would like something with a bit more oomph than .223.

I have found lots of articles on sub-sonic .300 performance through sbr and silenced ars, but I haven't found much on how .300 performs through longer barrel lengths with super-sonic ammo.
Any insight would be appreciated.  

Thanks in advance for helping out the new guy.

these types of threads always end up in the my dogs better than your dog bin. I would build a 5.56 rifle first simply for the economy of the ammo and it being so common . then If I wanted to hunt with an ar I would build an upper for that in an alternate caliber .

To answer your question before all the madness starts 300 supersonic performs on par with 7.62x39

300 BLK Info

Ballistics Data (supersonic, 16" barrel)

I have two 300 BLK's and that being said I only shoot supers and both are carbines..........16 inch........Ammo is pricey.......but I reload......I think its a great carbine cartridge....and have taken deer with it..............strictly for paper punching.......can get expensive if you are buying ammo..........depends on what you want it for............

Welcome.

#1 rule. Build what you want.

# 2 rule. Get both!

How much do you plan to shoot a year? A lot, then have a 556 upper & another better one suited for hunting.

That will lead to another lower, then some more parts then another rifle, or pistol, or...

Do you reload?

My advice is to build or get a 5.56 first.

Everyone who has an AR should have at least one chambered in 5.56.

Then build another upper down the road in 6.8, 6.5 or 300.

If commonality, the ability to shoot subsonic and a step up in power are your main concerns, get a 300, but if you want an even further step up in performance, get a 6.8 or a 6.5.

my .02c, worth what what you paid for it.

Personally I wouldn't go any longer then 12-1/2" for a 300 BLK. The reason you don't hear much about 300 BLK in longer barrels is because there is a steep velocity drop off in longer barrels. Going from a 10" barrel to 16" gains you less then 180 FPS (supersonic). The weight is not worth the little performance gained.

Build what you want

16'' rifles are a lot less hassle. No stamp, no $200 extra fees, no problems crossing state lines, etc.
It's 4'' of barrel, not the end of the world.

For your first AR go with a 5.56, but ensure you upgrade your lower to have a good trigger.

For your next build put together an upper. Then you can just swap uppers to the different caliber.

I vote 6.8 SPC for the second gun though. When comparing the supersonic 300 BLK and 6.8 SPC the 6.8 is the clear winner.

6.8 ammo is plentiful on the internet if you can't find it locally. Right now ammo for both are priced about the same. So, might as well go with more capable 6.8.

Eventually you'll want to get into reloading.

I'm only now looking at building a 300 BLK now that I have a suppressor in jail.

I didn't say it was a big deal to other shooters or to myself. I was explaining to the original poster why most guys don't choose longer barrels in the 300 BLK. Sure, there are advantages to having a 16" barrel (I'm sure he already knew that and so did I). He wanted to know why many don't go that route.