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If you have a front sight, the light/laser will be obstructed if you mount this behind the front sight. The height is 1.4 inches which means if you want to completely avoid seeing the unit in your field of view, you will want to use an optic mount that is a lower 1/3rd co-witness height of 1.6 inches or taller.
I have this unit mounted on the top rail of my AR pistol, fitted with aluminum Ultradyne C2 flip up sights, just behind the front sight (one slot behind the sight in the down position). The unit is low enough that it does not interfere with iron sighting at all and the laser is unobstructed whether the front sight is in the up or down position. With the front sight flipped up, however, the light is almost totally obstructed and placing the front sight in the down position obstructs the bottom right quarter of the beam but it still shines plenty of light. Mounting it to the side with a MLOK-Picatinny rail section solved this problem, however I prefer the sight on the top rail ergonomically even with the partial obstruction.