Guns

Internal Cannon M61A1 Vulcan

The General Electric M61 Vulcan cannon finally went to war as an integral part of the Phantom with the advent of the F-4E, with the nose profile and APQ-120 modified to fit the weapon. Carrying 639 rounds in the internal drum, the six-barreled Gatling style rotary cannon provides a user-selectable rate of fire from 4000 to 6000 rounds per minute, delivering a muzzle velocity in excess of 3,450 ft/s, with armor piercing incendiary and high explosive incendiary round options available. Useful in both air-to-air and air to ground roles, the Vulcan was used for four confirmed kills by the USAF over Vietnam with the F-4E.

Accumulated gas is vented by ram air entering the Gun Purge Door on the nose of the aircraft. The door is held closed by hydraulic pressure and will automatically open to release the gas from firing the gun. It closes after the gas pressure has reduced below a threshold or the gun is de-energized, for example when switching Master Arm or all rounds have been fired.

💡 The venting process takes between 10 seconds to 1 minute, depending on the burst duration.

🔴 WARNING: Do not spool up the engines as long as ground personnel is working on the door. It will forcefully close automatically as soon as hydraulic pressure is obtained.

Employment

To utilize the M61, the GUN station select switch must be selected, the Master Arm must be ON, and the Gun selection on the Pinky Switch (Aft) must be actuated. This will illuminate the GUN lamp on the Head Up Display.

For air-to-air usage, the optical sight should be selected to A/A mode. In air-to-air mode, the optical sight functions as a lead computing optical sighting (LCOS), thus maneuvering in both elevation and azimuth relative to a radar locked target, or a default 1000' range setting in the event of no lock. With a lock, the range bar presentation is relative to 6700' just to the left of the tick at the 1 o-clock position, down to 1000' at the 6 o-clock position.

For air to ground use, the optical sight should be selected to A/G mode, the Delivery Mode knob should be set in the OFF or Direct mode, and the desired sight depression mil setting be entered for the intended attack profile. Depression is set relative from the Fuselage Reference Line (FRL). While Depression will lower the reticle in elevation, there is no azimuth drift due to relative aircraft motion.

Gun rate is controlled with the Rate switch on the Pedestal panel, and is selectable as High (6000 rounds per minute) or Low (4000 rounds per minute). Also on the Pedestal panel is the gun rounds remaining indicator.

Of note is the Auto Clear function; the cannon will fire approximately between 5 and 11 rounds from the point the pilot has released the trigger to clear all bolt actions in the cannon; this spin-down takes approximately one second, and the gun cannot be fired again during this operation. While the Pedestal panel carries the Auto Clear switch, this switch does not have any control over the internal M61A1 cannon, and can only deactivate round clearance for podded external cannon installations.

Sight Depression

For air to ground employment of the internal gun, the following sight depressions should be used.

External Cannon Pods SUU-16/A and SUU-23/A

Built to both provide gun capability to aircraft that did not mount them internally, and to increase available direct fire in the close air support mission, the SUU-16/A and SUU-23/A gun pods were both built around the M61A1 and a 1200 round ammunition load. Both pods are fixed-rate at 6000 rounds per minute, and up to three can be concurrently installed and driven by the fire control system; while up to five can be loaded for ferry purposes, and promotional images have shown such a configuration, only three can be activated.

💡 The inner wing pylons 2 and 8 are not wired to operate the pods. Only the center station and the outer wing pylons can fire the guns.

The primary difference between the -16/A and -23/A pods are their method of power; the former using a ram air turbine arrangement to drive its motor, while the latter is internally powered. This difference had an effect on relative utility - while the -23/A could be fired at any airspeed, thus allowing it to be loaded onto slower aircraft, the -16/A's ram air turbine required a minimum speed of 300 knots to fire, and only reached its optimum performance above 400 knots. Both pods have an auto-clear function for safety, which cause rounds loaded in the breeches to be released back into the ammo bin, which does entail a momentary delay of one to two seconds prior to being able to fire the pod again.

External Employment

Activation of the SUU pods are performed by selecting their respective Station Select buttons, and placing the Delivery Mode knob in OFF or Direct. The Weapon Selector knob can be in any position other than TV or ARM, unless CAGE mode is activated. The desired Auto Clear setting should be selected, and Master Arm set to ON to activate the pod. On the SUU-16, the Master Arm activation deploys the ram air turbine.

Installed, the guns are boresight along the Fuselage Reference Line. Thus, with the optical sight in air to air mode, the rounds will fall in accordance with reticle center, just as the main cannon- only with the natural offset of their respective pylon position, thus allowing for effective natural gunnery, if only affected by a moderate increase in resulting CEP. In the same fashion, in air to ground mode, the SUU pods observe the same performance relative to the reticle depression schedule as the internal cannon, again, with their respective offsets from centerline and CEP increase.

To safe the gun pods after combat, prior to deactivating Master Arm, the Auto Clear switch should be placed into the Auto Clear position, the trigger squeezed momentarily, and the respective Station Select buttons pushed to off. On the SUU-16, this will close the ram air turbine.

External Sight Depression

For air to ground employment of the external gun pods, the following sight depressions should be used.