Smoke Composites makes some truly lightweight buttstocks for AR-15s. Their Carbon Fiber Buttstocks weigh between 94 and 128 grams (3.3 – 4.5 ounces) depending on the model. Those weights include the stock with integral buffer tube. They are available in two standard lengths and with an open or closed “shoulder plate”. They can also provide a custom length for a fee. These stocks use carbine buffers and springs.