A gas piston-operated rifle including a bolt carrier and receiver assembly, comprising: a gas piston operating system including a gas piston actuated by discharging the rifle and a solid transfer rod operably coupled to the piston for axial movement; a receiver defining an elongated internal cavity having inner sliding surfaces; a generally cylindrical bolt carrier slidably disposed in the cavity of the receiver for axial reciprocating movement, the bolt carrier including: a front end and a rear end; a front supporting section proximate to the front end, the front supporting section having guide rails (321, 322) sized and configured to engage the sliding surfaces of the receiver; a rear supporting section proximate to the rear end, the rear supporting section being sized and configured to engage the sliding surfaces of the receiver; a reduced diameter middle portion disposed between the front and rear supporting sections (320, 330), the middle portion being sized and configured to prevent engagement with the sliding surfaces of the receiver when mounted therein; and a key protruding outwards from the bolt carrier and defining a forward-facing thrusting surface positioned to abuttingly engage the transfer rod for reciprocating the bolt carrier, wherein the guide rails (321, 322) of the front supporting section do not extend rearwards beyond the thrusting surface of the key; wherein the bolt carrier is operable in response to discharging the rifle to travel from an unactuated forward position to a rearward actuated position without the middle portion of the bolt carrier engaging the receiver. a gas piston operating system including a gas piston actuated by discharging the rifle and a solid transfer rod operably coupled to the piston for axial movement; a receiver defining an elongated internal cavity having inner sliding surfaces; a generally cylindrical bolt carrier slidably disposed in the cavity of the receiver for axial reciprocating movement, the bolt carrier including: a front end and a rear end; a front supporting section proximate to the front end, the front supporting section having guide rails (321, 322) sized and configured to engage the sliding surfaces of the receiver; a rear supporting section proximate to the rear end, the rear supporting section being sized and configured to engage the sliding surfaces of the receiver; a reduced diameter middle portion disposed between the front and rear supporting sections (320, 330), the middle portion being sized and configured to prevent engagement with the sliding surfaces of the receiver when mounted therein; and a key protruding outwards from the bolt carrier and defining a forward-facing thrusting surface positioned to abuttingly engage the transfer rod for reciprocating the bolt carrier, wherein the guide rails (321, 322) of the front supporting section do not extend rearwards beyond the thrusting surface of the key; wherein the bolt carrier is operable in response to discharging the rifle to travel from an unactuated forward position to a rearward actuated position without the middle portion of the bolt carrier engaging the receiver. a front end and a rear end; a front supporting section proximate to the front end, the front supporting section having guide rails (321, 322) sized and configured to engage the sliding surfaces of the receiver; a rear supporting section proximate to the rear end, the rear supporting section being sized and configured to engage the sliding surfaces of the receiver; a reduced diameter middle portion disposed between the front and rear supporting sections (320, 330), the middle portion being sized and configured to prevent engagement with the sliding surfaces of the receiver when mounted therein; and a key protruding outwards from the bolt carrier and defining a forward-facing thrusting surface positioned to abuttingly engage the transfer rod for reciprocating the bolt carrier, wherein the guide rails (321, 322) of the front supporting section do not extend rearwards beyond the thrusting surface of the key; a rear supporting section proximate to the rear end, the rear supporting section being sized and configured to engage the sliding surfaces of the receiver; a reduced diameter middle portion disposed between the front and rear supporting sections (320, 330), the middle portion being sized and configured to prevent engagement with the sliding surfaces of the receiver when mounted therein; and a key protruding outwards from the bolt carrier and defining a forward-facing thrusting surface positioned to abuttingly engage the transfer rod for reciprocating the bolt carrier, wherein the guide rails (321, 322) of the front supporting section do not extend rearwards beyond the thrusting surface of the key; wherein the bolt carrier is operable in response to discharging the rifle to travel from an unactuated forward position to a rearward actuated position without the middle portion of the bolt carrier engaging the receiver.