What is the highest muzzle velocity of a gun on any fighter aircraft?
The muzzle velocity of guns is based on multiple factors and generally sees a wide range. Most importantly how the bullet is loaded (gun powder wise) and what mass the bullet is. For example the P-51 mustang carried 6 50 caliber BMG's (M-2). Which (although not available at the time) is capable of accepting the Caliber .50, Saboted Light Armor Penetrator, M903 round which has a quoted muzzle velocity of 4,000 ft/s (1,219 m/s). It should also be noted that the base muzzle velocity of an M2 is 2,910 ft/s (890 m/s) for M33 ball round. So it really depends what round you are loading in.

The F-35 Joint Strike Fighter has the GAU-22 Gatling Gun which with the TP HEI ammunition has a quoted muzzle velocity of 3,560 feet (1,085m) per second (15 m/s faster than the GAU-8 on the warthog). 


(Not sure about you guys but I often test my gatling guns with my multi-meter...)
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