A pretty good hint was in the Notice of Intent:SIG has won a contract to provide the Rattler as a PDW for SOCOM use.
Special Operations Command finally found its next-generation personal defense weaponAfter five years of searching, U.S. Special Operations Command has settled on Sig Sauer's lightweight PDW.taskandpurpose.com
5.56 or .300 BO. It will be interesting to see which muzzle device is selected for suppressed use.
Since the emphasis seems to be on noise (with the 300 BO) and concealability , then a compact SLH would seem likely.The Government hereby submits a notice of intent to award the Commercial Personal Defense Weapon (PDW) contract to Sig Sauer. This IDIQ will provide complete PDW weapons (Sig Rattlers - 5.56mm and .300 Blackout caliber) that includes suppressors (SL series), cleaning kits, magazines, quick barrel change kits and force on force training kits. Furthermore, parts, sustainment, and New Equipment Training will also be part of this IDIQ. The PDW system will allow Operators to have maximum firepower in a concealable weapon.