An increasingly popular example of that is a skeletonized lower receiver, which is a lower with a significant percentage of the material removed so that what remains is a skeleton of a lower. like this one from our friends at F1 firearms. These are extremely lightweight and good for competition builds or race guns. If you're choosing a lower receiver based on how sweet it looks, or how lightweight it is, you're probably going to want to check out a skeletonized lower.