Ive built two AR pistols now. A 8.5 inch 5.56 and a 4.5 inch 9mm. Ive got the itch for another before someone puts the skids to them too. I'm thinking 300 blackout for a couple reasons. One is in a shtf senerio id allways have cast bullets I can feed it and just the price of 223 ball bullets keeps me from going out and blasting 500 rounds a day out of my 5.56 guns. Kind of the same reason I built the 9mm. I can shoot 5 or 6 9mms for the cost of one reloaded 223. Now the 300 also has an advantage over the 9 in that it could be used as a deer rifle. Many claim that with a 110-125 bullet they do well out to a 100 yards. I got looking around at loading data and found what we did was come full circle. My first deer rifle was a 30 carbine. I shot maybe a dozen deer with it and for the most part feel I got away with it. All shots were 50 yards or less in stands set up I the swamp. Most at about bow hunting range. Another old timer at camp hunted exclusively with one and killed truck loads of deer but thinking back I can remember some we spent a lot of time tracking some we didn't recover and he was the type that many times had a few to many in his blind and I'm sure he shot deer that ran off and didn't even bother much more then looking where he shot to recover them. I guess what I'm asking is the TRUTH from guys that have done it. Not ballistics experts or theorists. A litte 8.5 inch deer gun with a arm brace would be an awful handy hunting package if it worked. I know a 10.5 would be better but Michigan law is funky with overall length and a 10.5 is pushing it to the point its to long to be considered a handgun. In the real world am I getting any advantage over using my 8.5 inch 5.56 with partitions. It would be real nice to here from someone who has ACTUALLY DONE IT WITH BOTH.