FEATURUING SPECIAL GUEST, SUTURE.
The rail usually referred to as the Picatinny Rail, named after the Picatinny Arsenal Facility.
Modern guns now come standard with rails. Sometimes rails are attached onto a gun that usually wouldn't have one in order to accessorize it.
Free floating barrels have the advantage of not being affected by the foregrip if it it distorts due to the environment.
Those who have played Modern Warfare 2 or Metal Gear Solid 4 should be familiar with all of this.