So...the front sight is part of the gas system then?
This is usually on the AR's with the A2 front site post. This is not always the case as seen in the pic above, and also if you go with an upper without a A2 post and a longer handguard/quadrail.
An example the AR15 on the left has the standard A2 post with the shorter sight radius, while the one on the right has a 13-inch handguard (no A2 sight) which would allow a longer sight radius if you choose to install back-up Iron sights.
Both are 16-inch barrels but would have different sight radius lengths.