I have a couple of handmade Haversacks....I've used them as possible bags in the past. In my opinion, a shooting bag needs to be large enough to hold the items that are regularly used in shooting and also include a few simple tools needed to mainatin the firearm in good working order. All that will actually fit into a surprisingly small bag.
A haversack is used to carry all the other possibles, along with a few food items, that one might take on a trek into the woods. Haversacks are larger and roomy so as to better accomodate these sometimes bulky items....and there-in is the problem.
Proper shooting bags need to have a space limitation, so that a person that shoots from the bag can easily (mostly by feel) find the items that are needed to shoot the firearm from the bag. The capricious space of a haversack, compared to a shooting bag, fights this fluid process by allowing items (especially small items) to easily get tucked into corners, or mixed at the bottom of the bag, or lost among bulky items.
Give a shooter space and he will use it...the haversack will soon be full of all sorts of items and neat little do-dads and goo-gauhs. A few of these niffty trinkets even have a bit to do with shooting. Its human nature...if it fits take it. If you are always having to "look" into the bag while digging out your shooting items as needed...there are essential bag skills that will never be able to develop....
I will say this, "
Its a fine thing to watch an excellent rifleman shoot from the bag....a shooting bag."