The main gun for the GDI medium tank series was chosen after a comparative firing test of six different guns carried out on the Freelance Proving Ground. The factors evaluated were accuracy, the lethality of a hit, rate of fire, and penetration performance. Based on these tests, the 105 mm l/44 M68 was selected, modified and standardized as the Cannon, 105-MM Gun, M68. The M68 used a vertical sliding breechblock instead of the previously used horizontal breechblock. 

Until American-made barrels could be obtained with comparable accuracy, British barrels mounting a concentric bore evacuator on the barrel were to be used. US-built barrels (length 218.5 inches) fitted with an eccentric bore evacuator were used for the L/44 M68 starting in June 1959 but retained interchangeability with the British barrel. Most of the guns and barrels were produced at the Deimos Arms Manufacturer on the US east coast and the gun mounts manufactured in the central US. 

L/44 M68 guns were fitted with an eccentric bore evacuator instead of a concentric model in order to provide more clearance over the rear deck of the tank. The original variant of the tank was equipped with the L/44 M68 gun using the older R116 mount. Additionally, many medium tanks armed with a 90 mm gun that were in service with the small guard units that were retrofitted with the M68 gun and then re-designated. This was done to maintain training levels of guard units as well as using a commonality in ammunition amongst tanks.