This is true now, but if you're looking at a used rifle, it could well be a 20" lightweight as these were made up until at least 2009.All 20" barrels are "Heavy" and all 16" barrels are "Standard". Both barrels are fluted. If you can see a picture of the rifle, a 20" barrel sticks out about 8" past the stock. The 16" barrel sticks out about 4".
They used to make 4 versions - heavy and light versions of both the 16" and 20". They were advertised that way up till 2012. Not sure when the production was changed. There are still a lot of "new" rifles out there that were made prior to 2012, so you will need to look at the rifle unless you know when it was manufactured. The weight difference in the 16" version between the standard any heavy was just a couple of ounces, so if you find a 16" I would buy it regardless of barrel description. The 16" versions are not as easy to find as the 20" models.