Based on your description you have 3 inches with which to bullpup the thing, which can't be done what with that buffer tube thingummy.
Are you till-death-do-you-part committed to your present bag? Tennis/squash raquet bags offer quite some length and are also less noticable as being an object full of goodies. I'd recommend a banjo case but you may want to keep the friends you already have.
How much weight can you carry three times the distance you think you will need to travel to get home? (Doubling back, multiple changes of course, etc.) How far away do you expect to engage BGs? How much more proficient with the .300 BLK AR pistol are you than with a more traditional handgun?
My personal preference - and worth every penny that you have paid to read it - is that you want to move fast and light in a get-home situaton, avoiding/going around as much possible/potential trouble as you can. Small size (comparatively speaking), maneuverability, and the ability to transition back and forth between "obviously armed" and "it looks like I don't have a firearm, doesn't it") were the biases I used in selecting a Get-Home bag and what went in it.