According to the operators manual, the M110's typical rate of fire was 3 rounds per two minutes when operated at maximum speed, and 1 round per 2 minutes with sustained fire. The M110 featured a hydraulically operated rammer to automatically chamber the 200+ pound projectile. These rammers were prone to breakdown and generally slowed operation of the gun, because the rammers required crews to completely lower the massive barrel before using it. Highly trained and motivated U.S. Army crews could achieve 2 to 4 rounds per minute by using the hand-operated manual rammer, which was essentially a heavy steel pole with a hard rubber pad on one end. Using the manual rammer was physically demanding, but crews were not required to lower the massive barrels nearly as much as with the hydraulic rammer. Source: http://en.wikipedia.org/wiki/M110_howitzer