It looks like operators from the US Special Operations Command are looking into having a new PDW without having to reorient themselves to another weapons systems. They intend to purchase the SIG MCX Rattler PDW kits to convert existing M4A1 carbines. What this does is meet the USSOCOM's demand that for a compact weapon that can be chambered in both 5.56x45mm and .300 Blackout.

SIG Sauer already won the Modular Handgun System of the US Army in the form of the SIG P320 or the M17 (official designation).  According to The Firearm Blog, there are still contenders for the PDW requirement but for now time will tell if the MCX Rattler will hold up to the test.

Below is the announcement of the U.S. Army on this posted a FedBizOps:

The U.S. Army Contracting Command-New Jersey, on behalf of PM Soldier Weapons, intends to solicit, negotiate, and award a sole source contract to Sig Sauer pursuant to the authority of 41 U.S.C. 1901 as implemented by FAR Subpart 13.106-1(b). This contemplated sole source contract is for is for the purchase and delivery of the commercially available Sig Sauer MCX Personal Defense Weapons (PDW) Kits to satisfy the Government's requirement for Special Operation Command (SOCOM). The contemplated effort will be a Firm Fixed Priced (FFP) Purchase Order to be completed once the kits are delivered 14 days after award.

Sig Sauer was the only company identified through market research that could provide the necessary 10 each MCX Personal Defense Weapons (PDW) Parts and Kits which met the Government's requirements for a Commercial off the Shelf (COTS) conversion kit for the M4A1 to create a Personal Defense Weapon (PDW).

The acquisition of production items from sources other than Sig Sauer will not meet schedule requirements. Sig Sauer was the only company identified through market research that could provide the necessary MCX PDW Parts and Kits which met the Government's requirements for a Commercial off the Shelf (COTS) conversion kit for the M4A1 to create a PDW that meets the specifications identified in the sources sought notice posted to Federal Business Opportunities (FBO).

The U.S. Government anticipates award of one single purchase order as a result of this solicitation. Please note this notice DOES NOT constitute a Request for Proposals (RFP). However, any responsible source who believes to be capable of meeting the requirement may submit a capability statement, proposal, or quotation, which shall be considered by the agency, only if received by the closing date and time of this notice. A determination by the Government not to compete this proposed contract action based upon the responses to this notice is solely within the discretion of the Government.

The anticipated award date for this action is planned for 2QFY18.