Sun Weapon Skin for Gears of War Ultimate Edition
Unlocking the Sun weapon skin is only possible if you played day 5 of the Gears of War Ultimate Edition beta.
The Sun Weapon skin is a rare skin because it is only possible to obtain on one day of the Beta of Gears of War Ultimate Edition. In order to unlock it you would have participated in the Beta on this day.
It is unlikely that you will come across the skin very often in game, as there are so many skins and this skin in particular is quite difficult to obtain.
This skin is popular with current users as in Gears of War 3, the ability to obtain Gold weapons was highly saught after because you had to obtain Veteran Gear in Gears of War 2 in order to obtain the Gold Skin weapons for Gears of War 3!