Just got a "Ukoo Deluxe 24" Ukulele Bag" off Amazon for about $25, it fits my 8" barreled 9mm AR just perfect. Of course I had to go with flip up MBUS sights on it, having a fixed red-dot took up a bit too much space on my top rail. Still able to run a TLR-7 on it. At least if you plan on keeping it in a "ready to go" setup.
-If you don't care about breaking down your AR, then this shouldn't be as much of an issue.
24" Ukulele bag
8" barreled 9mm AR with SBA3 brace
Outside pocket fits 3, 33rd Glock mags inside of a basic condor SMG pouch that I have a sling-strap on.
It makes for a good quick access pack with a simple mag-bandolier for extra pew ability
I have used 'lawn chair' bags for carrying guns discretely to/from the range/car, but those are not exactly the best for actually carrying it around. Just short trips out of the house and putting guns into the truck, but for actually carrying something. I would want it to be fitted to the bag and the bag will need some semi-rigid padding to fill it out.