"Variants
M110A1, improved variant of the M110, fitted with a longer M201 203-mm / L37 howitzer. It entered service in 1971. Maximum range of fire was increased to 27 km. This self-propelled howitzer fires newly developed rocket-assisted, cluster projectiles, nuclear and chemical projectiles;
M110A2, improved variant of the M110A1. It entered service in 1978 and was fitted with double muzzle brake. Maximum range of fire was increased to 29 km."
Source: http://www.military-today.com/artillery/m110.htm