This high strength Nylon fiber weave backpack from Milspex is durable, practical and certainly looks the part. It is actually a double-backpack with the outer portion kept fast by a series of quick-release buckle tabs to separate the smaller portion resulting in reducing the overall size of the pack by about a third.
The main portion has ample interior capacity as well as a rear wall pocket for a camel pack but is otherwise a cavernous space for whatever you need to store with a couple of interior pockets for securing your smaller bits and pieces. The smaller outer portion is similar inside, with one continuous space but also includes and expander for almost doubling its height which includes a zip-up side for secure fastening.
The main portion is about 20 inches high (SMG sized); the smaller is about 18 inches which expands to approximately 35 inches (M4A1 sized). The main bag includes Velcro straps and patches for further securing or any Velcro attach patches and accouterments you wish to add.