I'm getting the urge to put together a AR15 pistol.  From actual experience could I get some input on barrel length (I'm looking at the 7"-8.5" range), and also difference in shootability between 300 aac and 5.56.  I reload both of those, but still, 5.56 is cheaper to shoot.  Are they quite a bit louder than a typical 16" barrel AR15?

Anything wrong with starting with the PSA complete pistol lower?

How about muzzle device? I see most coming with the A2 flash hider, but read that those that direct the flash forward are better for the shorter barrel.

i have the following ar pistols

4.5"  .22 it has an a2 birdcage when im not using a supressor
7.5" 5.56 kak flash can
10.5" 5.56 levang comp
7.5" 7.62x39 kak flash can
6.5" 9mm kak flash can

a short barrel 300bo will supress better than 5.56

waiting for my omega to come in than an osprey

psa pistol lower is gtg

5.56 gas could be as high as 18k PSI uncorking from a 7" according to an article I recently saw.
That is going to be loud/concussive.

With a linear comp, flash can or similar the sound is no greater than a 16" barrel.

There is no reason to spend a lot on a muzzle device.

For 223 a Levang comp works good and only $30. Levang

For 300blk I use a Kineti-tech on one and a BRT on another, but anything that directs the blast downrange should work fine.

A PSA Lower should work good.

300blk. More efficient use of the short barrel.

Both calibers shoot well in a short barrel but a 300 will likely be more efficient and if hunting was ever a thought a better option at the 8.5" length.  If only plinking I'd go 556.  The 300 would likely have slightly less concussion due to its efficiency and since you reload, it would be a good road for you to take.  

PSA would likely be fine and they will have sales happening in a week.

Muzzle device, any of the cones or others that direct down range will help.  If a suppressor is in your future go 300 for sure.