Abstract: Shown is a locking interface assembly that facilitates the integration of a commercial Long Gun Barrel tube with an existing gun mount of a conventional Tank. The interface assembly allows the non-conforming tube to be fitted to the mount in a precise fashion, eliminating clearances between the mount and tube, permitting re-tightening of elements that become loose during high inertial loadings. The interface assembly is comprised of a thrust nut, bearing, retaining ring, king nut, adapter, pin, nut key, locking pin, retainer pin, retaining ring key, key retainer pin and screw. The retaining ring is slid over the tube, followed by the a coupling subassembly comprised of the bearing and the thrust nut. The coupling subassembly engages the tube shoulder to prevent the bearing from rotating. The king nut engages the tube to capture the bearing against the tube shoulder and to prevent the bearing from moving with respect thereto.