Backup Iron Sights (BUIS) come in many different shapes and sizes. Because of this, it is important to know the differences between them so that you choose the correct one for your AR. Keep reading to learn about the differences.
Front BUIS fit into two main groups - forearm rail height (Fig. 1, aka "same plane") and gas block rail height (Fig. 2). The differences between the two become pretty clear when you see the two side-by-side.
Forearm rail sights, like the Midwest Industries Flip-Sight for Forearm Rails, are designed to be installed on the top of a railed handguard, such as a quad-rail. These sights will line up with virtually all rear BUIS. Gas block sights are slightly taller than forearm rail sights in order to make up for the fact that gas block rails are usually shorter than most quad-rails and upper receivers. A few choices of gas block sights include: the UTG Quick-Detach Fixed Front Sight, the UTG Tactical Flip-Up Front Sight, the ProMag Flip-Up Front Sight, and the Midwest Industries Flip-Sight for Gas Blocks.
Rear BUIS fit into two main groups - flip-up and stationary. A solid choice for a stationary rear sight is the UTG Complete Rear Sight. A few choices for flip-up rear sights are the UTG Tactical Flip-Up Rear Sight, the Midwest Industries Emergency Rear Sight, and the GG&G AR-15 A2 Style BUIS. Rear sights are not height-specific, so regardless of your choice for a rear sight, it will work with the front sight.
You can mix and match your BUIS all you want, but just make sure you get the correct front sight for your specific AR setup.
(Note: Some gas block rails are the same height as the upper receiver and require the use of forearm rail sights!)