The gun could fire a 100 lb (45 kg) shell to a maximum range of 14 mi (23 km), with an estimated accuracy life of 1,500 rounds.

To address this problem a number of foreign heavy artillery guns were adopted, including the Canon de 155 mm GPF.

range of 25,000 yards (23 km), a vertical arc of fire from 0° to 65° (for comparison, GPF had only 35°), a projectile not exceeding 100 lb (45 kg) and the capability to be installed on a mount with either caterpillar tracks or rubber tires.

[2] A number of prototypes were produced in the 1920s (M1920 and M1920M1 were even standardized) and 1930s (a new design was started from scratch after 1929), the projects were repeatedly put on hold due to lack of funds.

[3] Developed in the summer of 1930, the radical split-trail carriage for both 155-mm gun and 8-inch howitzer designated T2 was the first in the US to feature an all-welded construction as well as a 8-wheel 2-axle roll-bearing bogie for high-speed mobility.

[5][failed verification] The new gun design used a barrel similar to the earlier 155 mm GPF, but with an Asbury mechanism that incorporated a vertically-hinged breech plug support.

Once on the ground, the limber-end of the trail legs were separated to form a wide "vee" shape with its apex at the center of the carriage pivot point.

[11] The carriage M1 and M2 were shared with the 8-inch (203 mm) Howitzer M1, differing only in the gun tube, sleigh, cradle, recoil and equilibrators, weight due to the heavier barrel.

[12] The Long Tom saw combat for the first time in the North African Campaign on 24 December 1942, with "A" Battery of the 36th Field Artillery Regiment.

A small number of Long Tom guns were authorised for supply via Lend-Lease channels, to the United Kingdom (184) and France (25).

[18] 155 mm gun motor carriage T79, based on T23 medium tank chassis, never advanced past proposal stage.