The SW40 pistol in Battlefield 4 is patterned after the real Chiappa Rhino is easy to unlock, and many have probably done it by accident. To unlock the revolver you need to complete the Curve Ball assignment. To do so kill ten enemies with impact grenades, and that's it.
Once you completed the assignment you will be rewarded with a strange looking gun that has similar stats as the REX. Both SW40 and the Rex do the same amount of damage, and have other similar stats.