The article below might help with actual facts on barrel length. If you have a "shit ton" of 5.56 you might want to save the brass and get into reloading/300BLK at the same time. Reloading does not have to be expensive to start with and that brass can easily be turned into 300BLK.

http://www.dtic.mil/ndia/2010armamen...hilipDater.pdf

After researching it and from personal experience, I don't don't see the point of a 5.55/.223 barrel that short. Why have it that short over a standard pistol? Is it the lethality, concealability, accuracy, cool factor? This does not mean others are wrong because they like their AR that short. I am just not in that camp. Here is my thinking on it.

Usability/Lethality: It spikes pressures and beats the hell out of the barrel and you (or a supressor if used). It does not give the bullet enough force behind it (thus wasting a majority of the powder unlike a cartridge such as the 5.7x28) and renders the gun less effective than a 'real' pistol. The .223/5.56 cartidge was built for a 20" barrel. At 8.5" barrel length is less than half what the cartidge was intended to be. The round count? Maybe, but there are other 'real' pistols like the FiveSeven with standard 20/30rds or just a 30rd mag for say a Glock/92FS and much more accurate at 100yds.

CCW: AR pistols (defined here as having a barrel under 10.5". This is not the legal definition, just mine for this discussion on short barreled guns, pistol vs. rifle length. I think 10.5" and longer get into being more of a rifle) are too bulky to be easily concealed (yes this is debateable, I am thinking more along the line of walking through a mall like a normal person not CBQ) and more difficult to control (until your stamp comes back and you can put it in the SBR configuration above). If in CBQ mode, the military obviously likes a standard of 14.5". I would pick my 10.5" 300BLK easily for CBQ drills, as Ray1970 is moving towards.

Accuracy: I have a Sig Swat in 5.56 at 10.5" still with a pistol configuration, BCM AR upper at 14.5" in 5.56 and a custom 300BLK at 10.5" among others. I have two SBRed lowers, one in a pistol config and one in rifle. This allows me to quickly build-a-gun for how I feel at the time. All have their uses but the Sig is the least effective IMHO even at 10.5" and not your 8.5". The other two uppers are much better in either configuration. 300BLK is awesome supressed and handles much differently than the Sig at the same length to me.

Reloading: Shoot the "shit ton" of 5.56 you already have. Take the brass and cut it for 300BLK. I use a $25 Harbor Freight mini saw myself. You can get a press kit with most everything needed to get started for around $150. Die set for $30-50. Powder, primers and bullets are a given. After your first 1000 rounds of 300BLK it all pays for itself. If you want, you are welcome to come over and I can show you how I do it. It is easy but can take some time.

PS:My research found that at 10.5", the 300BLK uses all of the powder where at 8.5" it 'may' use all the powder depending on the cartridge.

Cool factor: they are guns.... They are all cool.

I sounds like you may have already thought of most if not all of this, thus this thread. You could easily rebarrel to 14.5" for now to create the brass. That would be a win-win right there.