AUG is the best bullpup design on the American market. As said, it's not modular, you can't change it much, like a HK gun, it is what it is.
The bullpup concept has a lot of different attributes, which affect use and handling. Few compare them to other carbines, but a little thought brings it out. The optic setup is required on them, because iron sights are so short and inherently less accurate. That optic isn't easily changed unless the mount is railed. The magazines are proprietary, AUG only, and fast reloads three gun style not much possible.
Point being, it sacrifices a lot to put the action as far back into the butt as possible. That means the advantages of the traditional layout and having the controls, magazine, and barrel length forward are traded off. For example, which is better for CQB, a standard M16A2 20" barrel with bayonet, or the AUG with bayonet? One has a lot more reach than another, and standoff to strike the enemy is the paramount rule of combatives. It's the whole point of using air, missile, artillery, and crew served weapons long before they can respond with rifle fire under 500m.
As a sporting firearm for civilian use, not as important, which is why they sell better than in international defense circles.
