I just finished building a lightweight AR at 5 3/4 lbs w/empty 20 rd mag, Burris Fastfire2, and a 90 lumen light (not the brightest, but maybe the lightest REAL light). A buddy and I were just looking at it and trying to figure out if one can lower the weight any more without going to exotic materials. He suggested a lightweight boltcarrier. I have access to a machinist who coud probably get some weight off it for very little money. The question is: would a lighter boltcarrier require a lighter buffer or a heavier one? My guess would be a lighter one. Who has a good guess or (preferably) a physics/engineering background to help out?