The 155-mm Howitzer M1 was a short-barreled weapon using separate-loading ammunition. Over 4,000 of these guns were produced. It was first produced in 1942 as a medium artillery piece. It saw service with the US Army during World War II. The howitzer first saw action in North Africa in 1942. The 155-mm Howitzer M1 was developed as an afterthought, in the late 1930s, when the U.S. Army determined it needed a new medium field artillery piece to replace the WWI vintage Howitzer M1917/M1918. 

Extremely rare heavy Field Howitzer 155mm, 1944, deactivated with international demil certificate, very complete with all hard to find items,only for historic museum exhibion purposes. Please specify your full address and landline telephone number, probably the only historic exhibit available worldwide [www.milweb.net]

The new carriage was under development for much of the 1930s for the existing World War I era M1917/M1918 until 1939 when it was realized that it did not seem logical to put a new carriage underneath an obsolete howitzer. Consequently, development began anew with a carriage designed to be used for the 155-mm Howitzer. This was completed by May 15th, 1941 when the Howitzer M1 on the Carriage M1 was standardized. The howitzer itself differed from the older model by a lengthened barrel of twenty calibers and a new breech mechanism. Uniquely, it was the sole 'slow-cone' interrupted screw mechanism to enter service after 1920. This meant that two separate movements were necessary to open the breech, versus the single movement of the 'steep cone' mechanism that simultaneously rotated and withdrew the breech. 

American 155-mm howitzer M1 firing at German troops in the area of ​​Cherbourg [Via]

The 155 used "separate loading" ammunition comprised of four components: a projectile, a separate bagged propellant charge, a fuse and a primer. The propelling charge contained individual bags of powder, which could be reduced to adjust for range of fire. The projectiles weighed 95lb each. Fuses were placed in the base of the projectile after a ring used for shipping was removed. The primers were placed in the breech of the gun for firing.

It throws its projectiles at a muzzle velocity of 1,850 feet per second to a maximum range of approximately 16,500 yards. The rate of fire in rapid bursts is 3 rounds per howitzer per minute; for prolonged firing, 1 round per howitzer per minute. The M1 155mm howitzer typically fired 80% or more HE (high-explosive) shells. The howitzer could also fire smoke or white phosphorus shells.

 "Troops of a field artillery battery emplace a 155mm howitzer in France. They have been following the advance of the infantry and are now setting up this new position." , 06/28/1944 [Via]

The 155mm Howitzer Carriage M1 was of the single axle, 2-wheel, split-trail type. In traveling position the trails were locked together by a toggle type clamping mechanism and were limbered directly to the prime mover. The wheels were equipped with large pneumatic tires for high-speed transport. When limbered to a prime mover, the materiel could be drawn at speeds at up to 30 miles per hour on improved roads. The relatively moderate weight of the materiel added to its maneuverability and its ease of transport. 

The howitzer was fired from 3-point suspension, with the trails spread and the carriage resting upon an integral firing jack, the wheels being clear of the ground. In firing position, the howitzer had a range movement in elevation of 1,156 mils (65 degrees), and a traverse range of 942 mils (53 degrees), or 471 mils (26 ½ degrees), to the right and left of mid-position.

The tractor for the M1 155mm howitzer was the Diamond T truck, but the M5 high-speed tractor was also used when available. 

3-ton High Speed ​​Tractor, M5 (M5 HST) towing a 155mm Howitzer, M1 on the Route Nationale 13 (RN 13), circa June 1944 [Via]

Principal Characteristics of the 155mm Howitzer M1 and Carriage M1:

• Weight of 155mm Howitzer, complete: 3,825 lb.
• Caliber: 155mm or 6.102 in.
• Length (muzzle to rear face of breech ring): 149.2 in. (12.43 ft.) 
• Type of breechlock: Stepped-thread, interrupted screw
• Chamber capacity: 725 cu. in.
• Muzzle velocity: 1,850 ft. per sec.
• Muzzle energy: 2,260 ft-ton
• Maximum powder pressure: 32,000 lb. per sq. in.

Rifling:

• Length: 113.10 in.
• Number of grooves: 48
• Twist: Uniform, right-hand, one turn in 25 calibers
• Weight of projectile: 95 lb.
• Weight of powder charge: 5.94 to 13.86 lb.
• Maximum range with supercharge: 16,500 yd. (9.375 miles)

Rate of fire:

• Rapid bursts: 3 rounds per howitzer per minute
• Prolonged firing: 1 round per howitzer per minute

General Data Pertaining to the 155mm Howitzer Carriage M1:

Weights:

• Carriage, complete with weapon (without Howitzer cover and Accessories): 11,966 lb.
• Weapon: 3,825 lb.
• Recoil Mechanism M6: 1,582 lb.
• Spade: 184 lb.
• Firing jack float: 80 lb.
• Wheel, complete with tire and tube: 409 lb.
• Right shield: 79 lb.
• Left Shield: 86 lb.

Dimensions:

• Width of track, center to center of wheels: 82 in.
• Maximum width, traveling position (outside walls of tires): 97 3/8 in.
• Maximum height, traveling position (top of right shield): 71 in. 
• Length of howitzer and carriage, limbered: 26.5 ft.
• Road clearance (bottom of front trail stop): 14 in.