Even though the six 0.50-inch machine guns of the Sabre had a high rate of fire, one of the primary complaints by Sabre pilots was that these guns really didn't pack enough punch to ensure a kill of every MiG that got into their gunsights. The MiG-15 was actually a fairly robust aircraft, one which could sustain a considerable amount of damage and still keep flying. Colonel Glenn Eagleston submitted a report in which he estimated that as much as two-thirds of the MiGs hit by Sabre gunfire had actually escaped to return home and fight another day. A heavier cannon armament was clearly needed, but one which still preserved the high rate of fire of the machine guns which would give a higher probability of a kill during air combat.
It is a little known fact that some operational trials were actually carried out in Korea with cannon-armed Sabres. Four F-86E-10s (serial numbers 51-2803, 2819, 2826 and 2836) and six F-86F-1s (serial numbers 51-2855, 2861, 2867, 2868, 2884 and 2900) were pulled off the North American assembly line and fitted with a quartet of T-160 20-mm cannon and redesignated F-86F-2-NA. The T-160 guns were belt-fed and were capable of firing 1500 rounds per minute. The gun bays had to be completely redesigned and the guns had to be spaced further apart vertically with a totally new blast panel. The ammunition canisters could carry only 100 rounds each, for about 6 seconds of firing. The gun mounts had to be strengthened and the nose structure around the guns had to be beefed up in order to handle the extra amount of recoil. In order to prevent the buildup of gun gas in the cannon bays, where it could be an explosion and fire hazard, small doors were cut into the interior of the intake duct to extract the gun gas and suck it into the engine.
First tests were carried out with 51-2803 by test pilot George Welch over the Pacific firing range near Catalina Island. All test flights and gun firings were carried out at altitudes between 10,000 and 25,000 feet, with no problems being encountered. The remaining F-86F-2-NAs were delivered to the Air Force Armament Test Center at Eglin AFB.
The cannon-armed Sabre project came to be known as Project Gunval. Eight F-86F-2s were transferred to the 4th Wing in Korea in January 1953 for actual combat tests. The Gunval project was assigned to the 335th FIS, commanded by LtCol Vermont Garrison. Almost immediately, problems were encountered. In the very first aerial combats, the engines of the Gunval Sabres flamed out immediately when the cannon were fired, and no hits on MiGs were scored.
All of the *Gunval* Sabres had to be grounded to figure out what the problem was. It turned that during the firing of the cannon, excessive amounts of gun gas were being sucked into the engine, much larger amounts than the engineers had expected. The early stateside firing tests had been carried out at lower altitudes and no problems had been encountered, but at higher altitudes there was lesser oxygen to run the engine and the gun gas was causing a compressor stall, resulting in a flameout.
The idea of extracting the gun gas by sucking it into the engine had to be abandoned. The doors that bled gun gas into the engine intake duct were welded shut, and a selector switch was installed in the cockpit that permitted the pilot to be able to choose either two or four cannon firing. Small holes were drilled into the aft portion of the gun bay doors to alleviate gun gas buildup. These changes seemed to cure most of the gun gas buildup problems.
However, gun gas problems soon returned once combat trials were resumed. Test photos indicated that large amounts of gun gas were building up ahead of the nose during firing and were being ingested directly into the intake. This problem was ultimately solved by North American engineer Paul Peterson, who added a small horseshoe-shaped clip inside the recessed nozzle trough of each weapon. This clip broke up the gun gas, deflecting it away from the nose of the aircraft and trailing it harmlessly away in the wake of the aircraft.
This seemed to cure the gun gas ingestion problem, and combat tests resumed. A total of 282 combat missions were flown. Out of the 41 MiGs fired at, six were destroyed, three were probably destroyed, and 13 were damaged. Two *Gunval* Sabres were hit by MiG cannon fire, but both aircraft were able to return safely to base. The *Gunval* tests were completed on May 1, 1953, and the surviving aircraft were sent back to the USA, ultimately to be assigned to the Colorado Air National Guard *Minutemen* aerobatic team.
The combat tests were sufficiently encouraging that the T-160 cannon was placed into production by a division of the Ford Motor Company as the M-39, and the cannon ended up arming the F-86H and some of the Century Series of supersonic fighters.
Following the end of the Korean War, two F-86F-1-NAs (51-2916 and 51-2926) were fitted with Oerlikon 206RK 20-mm cannon and were redesignated F-86F-3-NA. Tests were carried out at Eglin AFB in April of 1954. The Oerlikon installation was not very successful--the guns were much heavier than the T-160s, resulting in a shift of the aircraft's center of gravity. In addition, the barrel life was too short and the stoppage rate was unsatisfactory. The F-86F-3s were assigned to Air Proving Ground Command for further tests in 1954-55, being redesignated JF-86F in 1957 and eventually scrapped.