Re: Rucksack as a rifle rest
When using an old school metal framed mountain ruck (known as a Large Alice Pack) we used to weld a metal bracket onto the bottom position of the frame and glue "iso mat" into the frame. This allowed your position to remain the same regardless of the contents of the pack.
Unfortunately I don't have a photo of the mount on an Alice Pack. It works very well and it is rock solid in the prone, particularly when your pack weighs over 80lbs.
Of course this was out of necessity because the M40A1 had no provision for a bipod, and we were not authorized to remove the front sling swivel and install a bipod stud on a Govt issued rifle. I shoot with bipods now.
Also, I was never overly concerned about shooting in the prone position anyway, because in almost every situation where a rifle would be employed, it was always sitting in a house on someones table/couch/pile of rubble in order to see enough of the target area and over ledges, walls, and through windows, etc. Nice, comfy, super stable prone positions are a luxury that rarely present themselves in real environments, particularly in urban environments.
I'll try to dig up a photo for those interested in trying it.