Very good points guys. Before I made this bag I looked at every BOB and GHB posted on this site and read links that others like Tim have posted. I like the idea of a trash bag to ditch some items if need be. Some items that are in my bag I might not need at all, but it seems like with the exception of the ammo or water the rest does not weigh much and might make a difference of a pound or so, which none of us will notice on our backs.
The cool thing Tim about my particular gun (when I get it back it's on a form 4) is that it if I want to I can leave it packed up. It's an AK Draco (with a straight receiver and straight sights) and it will have an under folding stock so it is only about 21 inches long if I remember correctly, and can fit into the backpack or I have an old tent bag I put it in so you can't tell what it is on the pack.
I am not too particularly worried about my vehicles breaking down but it could happen. I have Toyota's

But I guess it is possible. And if I am in my truck I am most likely camping or away from civilization shooting or four wheeling so this bag is good for that as well. Plus I keep a lot of items in my truck. I will post that some time as well.
Fishoid definitely has some great points. When I make my short range bag it will be a lot like what he is describing. I will use it for shorter range probably 15 miles or less, most likely will only carry my pistol, and I am hoping to keep the weight around 25 pounds including water which will be some canteens equaling 1 gallon of water. If I do carry a primary weapon as well then it will be an SBS with about 20-25 shells, so not a whole lot of weight there, but a bit larger at 23 inches long with the stock folded.
I just went for a jog/walk cause my fat ass is trying to get in shape and I am guessing water would go quick in this heat. I was outside doing my jog/walk for just over 20 minutes and i was pouring with sweat lol.
I have to respectfully disagree with your rule of 3 though. Water as you mentioned. I am guessing you could go one day maybe a day and a half without water if you stayed still for the most part, and maybe moved at night only. I have seen people go without food for more than 10 days and they could not even walk or talk coherently and they were just sitting in a room the whole time.
One thing all of us should do is test out our bag. I have gone on a couple of small hikes (to get in shape) with mine in the Superstitions, and around where I live cause there is plenty of desert lol. The weight for me was no issue, and I even jogged a couple times for about 100 yards. A test I will do but have not done yet is try to get home from a farther distance and try and live with what is in the bag and stay out for a night or two. This will give us a good idea of what you might need to add or take away from our bags.
If anyone would like to try this sometime let me know because I know I will try it out.