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The gun armament for the F-5A, F-5E, and the F-20 consisted of two M-39A cannon. These had been developed in the 1950's and produced initially in vast numbers for the F-100 Super Saber, the first supersonic fighter-bomber (1,395 built). The cannons were government-furnished-equipment - that is, they were provided from US government inventory. M-39 production had ended in the 1950's, and by the 1980's there were just over 300 shipsets remaining. This meant that either production would have to be restarted, or a different gun would have to be fitted.

The gun issue for the F-20 was a problem from the beginning. The 'engine change only' model used the F-5E forward fuselage and radar, so there was no issue. The original F-5G version of the F-20 offered to Taiwan in 1978 required a different radar in order to be capable of firing the AIM-7 Sparrow radar-guided missile. This was solved in the initial offering by deleting one of the M-39 cannon and using the extra space for the additional black boxes required for either a new Emerson radar or a modification of the F-16 AN/APG-66 radar with a smaller antenna. For the F-20, the new AN/APG-67 General Electric radar was fitted, with much larger and numerous black boxes, and a much larger antenna than the F-5E's Emerson radar. This required a new, larger radome. To accommodate all of this and still fit two M-39 cannon and the larger Panoramic Canopy, the F-20 was fitted with a completely redesigned fuselage forward of station 341.37 (aft of the canopy frame). The use of a 'shark nosed' radome actually improved the aerodynamics of the F-20, and suggested the designation Tigershark.

With South Korea looking like the launch customer by 1983, it was apparent a major radar modification would have to be made to meet the Korean's demand that the F-20 match the F-16's radar range against MiG-23 targets. This would require a larger radar antenna and more power. Elongating the fuselage or making any major change to the mold line was out of the question for aerodynamic reasons. So it was decided to move the radar installation aft so that the larger antenna would fit farther back in the existing radome. The equipment bays would have to be rearranged in the nose, and this meant either deleting one of the M-39 cannon or developing a new gun.

Aircraft guns were thought to be obsolete in the late 1950's. Major fighters of the period were developed without any gun at all (MiG-21, F-4). The Vietnam War, when fighters on both sides had to resort to just chasing the opposition around the sky without a weapon after all missiles had been fired and missed, proving the fallacy of that engineering judgment. The post-Vietnam fighter series (F-14 through F-18) were all equipped with guns. Logically these high-tech fighters would have been equipped with new-technology cannon. But all of these fighters were equipped with derivatives of a General Electric M61 Gatling gun, developed in the 1950's for the Century fighter series. Developing a new gun, it turned out, was really difficult. To design and test a complex, fast acting, pneumatic-hydraulic-mechanical device that would operate in horrendous vibration, low temperature, near-vacuum environments was a process that required countless thousands of hours of development and firing tests. Furthermore aircraft guns, although deemed necessary after all missiles had been expended, were considered low-tech, un-glamorous, and low-priority. The US Air Force had tried and failed to develop such a gun for the F-15 in the 1970's, and ended up equipping it and the F-16 with the M61. The Navy followed suit on the F-14 and F-18.

Bill McDowell, the F-20 project manager, was a card-carrying gun nut. He had been pushing for development of a new aircraft gun for years. By the 1980's a vast improvement on the M61 would be possible. A gun using caseless 30 mm ammunition would pack much more punch, be much more accurate, fit into a smaller space, and be less prone to jamming and mechanical problems than the M61. But he had the same problem convincing Northrop management of the merits of pursuing such a development that gun advocates in the services had in convincing the Pentagon. The Korean requirements finally provided the opportunity.

Ford Aerospace had acquired the old General Electric aircraft gun business in the 1970's. They were considered the sole source for a new gun for the F-20 (as is usual, management supports competitive bidding for subsystems, until it comes to a type of equipment in which they feel they have personal expertise. Then they just want to buy from the company that they 'know' is best). Ford, being by then integrated into the humungous automobile company, was immensely lethargic, bureaucratic, intransigent, and difficult to deal with - the antithesis of the usual F-20 co-investing vendor. Northrop was going to have to pay for the whole development, but get little in return in terms of royalties or data rights. Nevertheless, McDowell finally got his gun, and management probably took a 'we'll worry about that later' since the Ford subcontract would only result in significant liabilities if the South Korean or USAF ADF orders were awarded and the F-20 finally proceeded into production.

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