This ^^ I have a couple of them that I use.. One for regular hiking, one for fly fishing. I can keep the stuff in them that I need for those activities and just move the gun back and forth.
The one I use:
http://www.hillpeoplegear.com/Produc.../1/ProductID/5
Without PALs attachment points:
http://www.hillpeoplegear.com/Produc.../1/ProductID/2
Here's a smaller footprint one without the PALs as well:
http://www.hillpeoplegear.com/Produc.../1/ProductID/6
He's got the full size bag in this vid, the other bags above are a little smaller profile.