The document describes the SCAR modular rifle system which allows users to choose between a 5.56mm SCAR LIGHT or 7.62mm SCAR HEAVY configuration by switching out the barrel and bolt to the desired caliber while maintaining over 60% common parts and 100% ergonomic commonality from a shared upper receiver. The SCAR is presented as a potential replacement for multiple existing US military rifles with the SCAR LIGHT intended to replace the M4A1, MK18, and MK12 in 5.56mm while the SCAR HEAVY could replace weapons like the M14 and M110 in 7.62mm.