I know , the price is a bit over 100 but this is the grab n go bag I am looking at getting.
http://www.skdtac.com/S_O_Tech_Go_Bag_Extended_p/sot.506.htm
You can have an AR 15 rifle broken down to fit with plenty of room for mags !!! Plus you can add things with the Molle/pals webbing.
* A main compartment with double linked zippers (dimensions: 23" x 6" x 6")
* Internal hydration reservoir slot (canteen not included)
* 4 zippered flat side pockets for small items or documents
* 2 larger zippered external pockets
* MOLLE/PALS attachment webbing on both sides
* Padded contour single shoulder strap with quick-ditch side locking buckle
* Shoulder attachment points and carrying handles on each end
* Made in the U.S. out of military grade cordura cloth down to the webbing and fasteners to specifications of U.S. Special Forces
Here is a pic of one loaded out with various stuff, what you want in it is up to you.