We have been waiting to hear the results of this for a while! It is the replacement for the hand-fit Colt 1911s they currently have. They could not have gone wrong with the Springfield or Colt entry. However, congrats Colt!
SOURCE: http://soldiersystems.net/2012/07/16/sources-report-marsoc-purchase-1000-colt-1911a1-rail-guns/
The Colt Rail Gun: http://www.coltsmfg.com/Catalog/ColtPistols/ColtRailGun.aspx