The M109 is an American 155 mm turreted self-propelled howitzer, first introduced in the early 1960s to replace the M44. It has been upgraded a number of times, most recently to the M109A7. The M109 family is the most common Western indirect-fire support weapon of maneuver brigades of armored and mechanized infantry divisions.

M109 Howitzer
Kings of battle keep the fire; 1-9 FA fires its last rounds 140910-A-CW513-046.jpg
M109A6
Type
Self-propelled gun
Place of origin
United States
Service history
In service
1963–present
Wars
Vietnam War
Yom Kippur War
Iran–Iraq War
Western Sahara War
Persian Gulf War
Iraq War
2014 Israel–Gaza conflict
Iraqi Civil War (2014–2017)[1]
Yemeni Civil War (2015–present),
Saudi Arabian-led intervention in Yemen
Syrian Civil War
Specifications
Mass
27.5 tons
Length
30 ft (9.1 m)
Width
10 ft 4 in (3.15 m)
Height
10 ft 8 in (3.25 m)
Crew
4 (Ammunition handler, Gunner, Commander, Driver)
Shell
separate loading, bagged charge
Caliber
155 mm L/39 caliber[2]
Breech
interrupted screw
Traverse
360°
Rate of fire
Maximum: 4 rpm/3 min.[3]
Sustained: 1 rpm[3]
Effective firing range
Conventional: 18 km (11 mi)
RAP: 30 km (19 mi)
Main
armament
M126 155 mm Howitzer
Secondary
armament
.50 caliber (12.7 mm) M2 machine gun
Engine
Detroit Diesel 8V71T
450 hp (335.56 kW)
Power/weight
18.7 hp/t
Suspension
torsion-bar
Operational
range
216 mi (350 km)
Speed
35 mph (56 km/h)
The M109 has a crew of four: the section chief/commander, the driver, the gunner, and the ammunition handler/loader. The chief or gunner aims the cannon left or right (deflection) and up and down (quadrant).

The British Army replaced its M109s with the AS-90. Several European armed forces have or are currently replacing older M109s with the German PzH 2000. Upgrades to the M109 were introduced by the U.S. (see variants below) and by Switzerland (KAWEST). With the cancellation of the U.S. Crusader and Non-Line-of-Sight Cannon, the M109A6 ("Paladin") will remain the principal self-propelled howitzer for the U.S. for the foreseeable future.

link:https://www.simpleplanes.com/a/y9038M/R-Corp-M109-Paladin-Self-Propelled-Howitzer