SIG Sauer MCX Virtus Patrol in 5.56 NATO and .300 Blackout:
The SIG Sauer MCX Virtus Patrol in 5.56 NATO and .300 Blackout can be configured with whatever barrel length, handguards, pistol grips and buttstocks you want, and that's only the beginning of its modularity.
Source: gunsandammo.com