sentences:  It was, in turn, replaced by the BMD-1 beginning in 1969.Development of a new assault gun for the armed forces started at the OKB-40 design bureau of the Mytishchi Machine Building Plant (MMZ), under the supervision of chief designer Nikolaj Aleksandrovich Astrov.The first Ob'yekt 573 prototype was ready for factory tests in the second half of 1953.