Re: 5.11 Rush 12, 24 or 72 backback.
I have a 72 which I use as my gear bag for work (firefighter).
I also recently bought a Eberlestock Gunslinger II which I'm using now.
Between the two, the 5.11 Rush 72 is a better everyday pack IMHO. It just has more pockets for keeping all the junk I carry around more organized, and still carry a complete change of clothes.
One side pouch carries some survival components - paracord, caribeaners, small survival kit, and some EMS supplies. Other side pouch has a small camera, charger, ipod. Main compartment - knife sharpening kit full of DMT sharpeners and spare BM knife parts (screws, clips), and a spare change of clothes. Compartment flap over main compartment holds, pens, spare flashlight batteries, a couple knives, calculator and some paperwork.
All around I'd say the Eberlestock is a better made pack for serious use, but for a daily gear hauler I have no complaints with the Rush 72. Since I bought mine, they have improved the Rush 72 by adding additional lashing points to attach MOAB bag and a few other small improvements.
Here's some of the best prices I've found for 5.11 stuff -
http://www.manventureoutpost.com/brands/5.11,-Inc..html
This might work for your gun rest on your pack -
http://deltaoutdoorsman.com/scart/index.php?act=viewProd&productId=67