Only 100 rounds? a 550 Brick from walmart is light enough as it is.
I try to keep prepped, I have a BoB/ carbag loaded with 3 days food (yay for spam), water purif, knife, several hundred ft of paracord, a folding gerber shovel (fucking awesome BTW, works great for unstucking the car, and I am sure it would do alright making some urban jumps, although its a bit small for that), an extra set of clothes in there and some cash. Plenty of other little odds and ends too. It generally has a few spare boxes of ammo for whatever gun I am carrying too, generally 9mm. I dont ski with it on though, I dont need a piece of Iron to slam into strapped onto my body. I tried figuring out a way to do it, its just not possible though.