Of the several variations of the Mustang, the P-51D is considered the definitive version and was the first to have the bubble canopy. More "D" models were built than any other variant, with a total of 9,603 being produced. Armed with six .50 caliber machine guns, three in each wing, with 400 rounds for each inboard gun and 270 rounds for the other four, plus underwing pylons for auxiliary fuel tans or 1000lb bombs, the Mustang was a true fighter pilots airplane. At the conclusion of WW2, Mustangs were in operation all over the world, flying in several nations air forces. When the Korean conflict broke out, hundred of F (for fighters) 51's were in Japan awaiting scraping. Because they possessed too slow speed maneuverability that was absent in the new jets then entering service, the F-51 was ideal to work with Forward Air Controllers (FAC's) on the ground in Korea. The major drawback to this was the F-51's liquid cooled engine susceptibility to anti aircraft and small arms fire when attacking ground targets. This caused the Mustang to have the highest loss ratio of any aircraft in Korea. A total of 194 were lost in three years, with only 10 being downed in combat with other aircraft.