The base is brazed to a flat spot (having very slightly undercut ends, which get peened over the base ends) which is basically the same size as the base itself. The Armory did it so well that sometimes it almost appears to be an integral part of the barrel.

Most people attempting to duplicate the procedure - which must have involved some precise jigs and pretty heavy clamping - fall short (some are way short) of the desired effect. I wish you good luck!