The Vel 46 will have an unlock requirement similar to this, requiring you to reach level 55 before a challenge is revealed - once that challenge is complete, you'll have access to the gun in your own custom loadouts.
Based on the real-world MP7, the VEL 46 is a compact personal defense weapon designed for mobility and close-quarters engagements. It has been available in MW2 since launch but while the VEL 46 started out as an underappreciated and niche option it has since risen to prominence following a series of buffs.