I have a sneakybags ToT.
It normally carries my work laptop and crap, but it can also carry a SBR Uzi pretty easily. It can reportedly also take a broken down 10.5" AR, but I haven't tried that.
It's got webbing and loop stuff for mags, but I haven't done that either. Mostly because I want Velcro mag pouches so I can easily change stuff out and haven't found what I want yet.
Good bag, but a few things I don't like. The shoulder straps to make it a backpack are apparently sized for somebody much larger than me when cinched down all the way, and the hardware to adjust the shoulder strap doesn't have anything to grab the fabric of the strap, so it likes to slide around when you run. But nobody would ever give it a second glance.