Depending on the model and configuration, the length of an AR-15 can be different. When the stock is fully extended, a standard AR-15 with a 16-inch barrel and a collapsible stock will be about 32 inches long. If the stock is all the way down, the rifle will be shorter in length. Some AR-15 models may have barrels that are longer or shorter, which will change how long the rifle is as a whole. You should always measure your rifle to get a good idea of how long it is.