T20. The US started searching for the successor to the M4 in 1942. The T20 series experimented with different transmissions, suspensions, and armaments. The eventual M27 had a better gun and lower silhouette than the M4, but was not adopted as the M4 still performed adequately, and the army did not want to disrupt production. The project eventually led to both the M26 Pershing and the M24 Chaffee.