A quick coupling barrel retaining system for removable mounting of a barrel assembly to a receiver of a rifle, the barrel retaining system comprising: a barrel having a bore defining a longitudinal axis and an axial path for a bullet; a barrel extension having a front end coupled to the barrel and a rear end for coupling to the receiver of the rifle, the barrel and barrel extension collectively defining a barrel assembly; an annular shaped spring member coaxially mounted on the barrel assembly; a lock nut threadably engaged with the barrel assembly; a first radial spring seating surface disposed on the lock nut and being an annular surface facing in an axial direction; and a barrel nut removably mounted to the barrel assembly and having an end mountable to the receiver of the rifle, the barrel nut defining a second radial spring seating surface, the spring member being engageable between the first and second radial seating surfaces; wherein the spring member is positioned for compression against the first and second radial spring seating surfaces (549, 513) when the barrel assembly is mounted to the receiver of the rifle wherein the barrel nut comprises a plurality of splines engageable with a plurality of corresponding barrel locking lugs disposed on the barrel assembly, wherein the barrel assembly is rotatable in a first rotational direction to lock the barrel assembly to the barrel nut and further rotatable in a second rotational direction to unlock the barrel assembly from the barrel nut; wherein the lock nut is movable forward and rearward on the barrel assembly via rotating the lock nut, so that the first radial spring surface is axially adjustable in position. a barrel having a bore defining a longitudinal axis and an axial path for a bullet; a barrel extension having a front end coupled to the barrel and a rear end for coupling to the receiver of the rifle, the barrel and barrel extension collectively defining a barrel assembly; an annular shaped spring member coaxially mounted on the barrel assembly; a lock nut threadably engaged with the barrel assembly; a first radial spring seating surface disposed on the lock nut and being an annular surface facing in an axial direction; and a barrel nut removably mounted to the barrel assembly and having an end mountable to the receiver of the rifle, the barrel nut defining a second radial spring seating surface, the spring member being engageable between the first and second radial seating surfaces; wherein the spring member is positioned for compression against the first and second radial spring seating surfaces (549, 513) when the barrel assembly is mounted to the receiver of the rifle wherein the barrel nut comprises a plurality of splines engageable with a plurality of corresponding barrel locking lugs disposed on the barrel assembly, wherein the barrel assembly is rotatable in a first rotational direction to lock the barrel assembly to the barrel nut and further rotatable in a second rotational direction to unlock the barrel assembly from the barrel nut; wherein the lock nut is movable forward and rearward on the barrel assembly via rotating the lock nut, so that the first radial spring surface is axially adjustable in position.