So it begins. The Sig 320 is going to change the way we think about firearms. The EXO ONE (X01) is a patent-pending multi-caliber exoskeleton for the Sig Sauer P320 Fire Control Unit (FCU). In 20 seconds, you can change your 320 pistol into a legal, bigger, more capable pistol with no FFL and no tools. This is just the beginning.