Introducing the SIG556xi, rewriting the standard for modularity, reliability, and performance for the modern rifle platform. Based on the legendary SG 550, the SIG556xi will change the way military, law enforcement, and responsible citizens look at adaptable rifles. Utilizing the time-tested rotating bolt and adjustable gas piston operating system, the SIG556xi allows operators a choice of caliber, barrel length, handguard/stock configurations and lower receivers. A removable barrel allows the operator to select the necessary length for a given mission. Adding the appropriate bolt, the SIG556xi can be changed to any of three calibers, 5.56x45mm NATO, 300 Blackout, and 7.62x39mm NATO. Users can also quickly change out the non-serialized lower receiver, allowing the use of either AR-style STANAG magazines or rock-and-lock AK-style magazines. The SIG556xi upper receiver is the serialized part, making the lower an easy to purchase part. Polymer handguards accept all Magpul light and rail kits, while the aluminum and carbon fiber handguards accept 2", 4", and 6" M1913 rails. Truly ambidextrous, the SIG556xi features an ambi safety selector, magazine release, and user swappable charging handle (right- or left-handed).