The lightest grip i know of - HOG 13100 AR15 15D VERTICAL GRIP BLK

These grips withut doubt are the lightest grips I know of. If you're trying to make a sub 4lb. carbine, they're perfect for it. At 1.6 ozs, it's ighter than any other grip I found. The 15 degree angle may not be so comfortable for everyone, but I find no discomfort issue using the carbine with it and a sub 4lb. carbine you can carry around all day hardly realizing you have it with you! So, for your ultralight, you want this since the next lowest weight grip I found was over a half oz. heavier and when making an ultralight, evey 10th of an oz. savings in weight adds up.