More info about Ironsight
Even a tiny error in the angle of sight alignment results in a trajectory that diverges from the target on a trajectory directly relative to the distance from the target, causing the bullet to miss the target; for example, with a 10 meter air rifle shooter trying to hit the 10 ring, which is merely a 0. 5 mm (0. 020 in) diameter dot on the target at 10 m (33 ft) and with a 4. 5 mm (0. 18 in) diameter pellet, an error of only 0. 2 mm (0. 0079 in) in sight alignment can mean a complete miss (a 3 mm (0. 12 in) point of impact miss). At 1,000 m (3,300 ft), that same misalignment would be magnified 100 times, giving an error of over 300 mm (12 in), 1,500 times the sight misalignment. [note 1] Increasing the distance between the front and rear sights (called the sight radius or sighting line) helps to reduce eventual angle errors and will, in case the sight has an incremental adjustment mechanism, adjust in smaller increments when compared to a further identical shorter sighting line. With the front sight on the front end of the barrel, sight radius may be increased by moving the rear sight from the barrel onto the receiver or tang.