to 64 projectiles (although the propellant stowage allowed a maximum of 36 charges for APDS rounds). The gun was fully stabilised with a fully computerized integrated control system. The secondary armament consisted of a coaxial L8A1 7.62 mm machine gun and another 7.62 mm machine gun mounted on the commander's cupola. An advantage of using two-part ammunition was that in the case of inert rounds like APDS the loader could reach for the next round and hold it in his lap, ready to load while the gunner was acquiring the target and firing. This practice increases the rate of fire but would be hazardous with one-piece ammunition. Chieftain had an NBC protection system which Centurion lacked. An infantry telephone was fitted to the rear of the tank to facilitate communication with infantry. The initial Fire-control system (FCS) was the Marconi FV/GCE Mk 4. A .50-inch (12.7 mm) ranging gun was mounted above the main gun (with 300 rounds available). This fired ranging shots out to a maximum of , at which point the tracer in the ranging rounds burned out although the high explosive tip still created a visible "splash" on impact. The tank commander had a rotating cupola with nine vision blocks -giving all round view, plus the 7.62 mm machine-gun and an infrared (IR) capable projector coaxial with the weapon. The aiming systems were provided for both the gunner and the tank commander; they had 1x or 10x selectable magnification power, increasing to x15 in the Mk5 and beyond, and they were replaceable with IR vision systems for night operations (3x magnification power). The commander could rotate his cupola to bring his sight onto a target and then engage the mechanism that brought the turret round on to the correct bearing so that the gunner could complete the aiming. The commander's controls had over-ride capability on those of the gunner. The left side of the turret had a large searchlight with an electrically controlled infrared filter inside an armoured box, with a relatively long range – up to . From the beginning of the 1970s, the Mk 3/3 version replaced the ranging gun with a Barr and Stroud LF-2 laser rangefinder with a range. This allowed engagements at much longer ranges, and also could be linked to the fire control system, allowing more rapid engagements and changes of target. On later models, fire control was provided by the