The downside to adjustable sights is the inherent fragility of the moving parts. A fixed sight is a solid piece of metal, usually steel, and if firmly attached to the gun, little is going to be able to damage it beyond usefulness. Adjustable sights, on the other hand, are bulkier, and have parts that must move relative to the gun. Solid impact on an adjustable sight will usually knock it out of adjustment, if not knock it right off the gun. Because of this, guns for self defense or military use either have fixed sights, or sights with "wings" on the sides for protection (such as those on the M4 carbine).