Unit of Fire

There are some military concepts from the time of the Pacific War that crop up frequently in contemporaneous accounts, yet remain sources of confusion into the 21st century. One such concept is the unit of fire.

The unit of fire was defined by the U.S. Army as follows (Army Field Manual 9-6, "Ammunition Supply", 15 June 1944):

A unit of measure for ammunition supply within a theater from a tactical point of view, based upon experience in the theater. It represents a specified number of rounds per weapon, which varies with the types and calibers of the weapons. The unit of fire is not synonymous with the term "day of supply".... In general, it represents a balanced expenditure by the various weapons under conditions of normal action. The unit of fire prescribed by the War Department may be modified by theater commanders as necessary for each individual theater.

It differed from the day of supply, defined as follows (ibid.):

Estimated average expenditure of various items of supply per day in campaign, expressed in quantities of specific items or in pounds per man per day. A day of supply for ammunition is expressed in rounds per weapon per day.

Accounts of troops in combat written during or shortly after the war make frequent use of the term "unit of fire", but the term "day of supply" is found almost nowhere outside of logistics manuals and reports. This is unsurprising. The day of supply was used for high level logistics planning purposes, such as determining the desired rates of ammunition production and the allocation of shipping to each theater, while the unit of fire was used in tactical planning by combat officers closer to the front. The unit of fire was not regarded as a rigid allowance: A combat formation might expend several units of fire in an day of extremely heavy combat, while a formation encountering only light resistance might use much less than a unit of fire per day.

It is clear that troops understood the unit of fire to be roughly the amount of ammunition consumed in a day of heavy combat. Eugene Sledge, a Marine mortar crewman in the Pacific, certainly understood it in this sense (Sledge 1981):

Determined from experience, a unit of fire was the amount of ammunition that would last, on average, for one day of heavy fighting. A unit of fire for the M1 rifle was 100 rounds; for the carbine, 45 rounds; for the .45 caliber pistol, 14 rounds; for the light machine gun, 1,500 rounds; and for the 60mm mortar, 100 rounds.

The Marine Corps history also uses the term in this sense (Garand and Strowbridge):

The IIIAC assault elements and accompanying garrison forces were directed by Headquarters, Expeditionary Troops, to carry with them sufficient rations for 32 days, water enough for 5 days when pro-rated at 2 gallons per man per day, medical supplies to last 30 days, and a 20-day supply of clothing, fuel, lubricants, and miscellaneous equipment. For the assault phase, all weapons would be allowed five units of fire--a unit of fire being that amount of ammunition which CinCPOA had determined from previous campaigns would last for one day of heavy fighting.

The unit of fire was specified as a number of rounds per weapon. However, combat troops often spoke of a unit of fire for a platoon, company, or other combat formation (Sledge 1981):

We set up our two mortars in a large crater near the now knocked-out pillbox and registered in the guns for the night. The ammo carriers dug into the softer coral around the edge of the crater. An amtrac brought up rations and a unit of fire for the company.

A unit of fire for a company would be the sum of the units of fire for every weapon in the company. This was, in fact, one of the reasons for defining a unit of fire. Instead of a company commander having to request each ammunition type separately after adding up the needs of every weapon in his unit, the company commander could simply request units of fire for his company. He would then receive a precalculated (and often prepackaged) load of ammunition whose composition was a reasonable estimate of the actual expenditures by different weapon types. This greatly simplified managing the ammunition resupply. The down side was that, if a particularly weapon type was being used less than usual due to some peculiarity of the kind of combat being engaged in, the company would gradually accumulate an excess of that ammunition type. The resulting potential for wastage was considered an acceptable tradeoff for simplified logistics.

The following table, taken from the U.S. Army's official history of production and logistics (Leighton and Coakley 1955), illustrates some of the differences between the unit of fire and the day of supply.

U.S. Army units of fire

For small arms, the day of supply was much less than the unit of fire, reflecting the fact that combat was a relatively infrequent occurrence in a soldier's life. It also reflected the generous American division wedge, in which a large fraction of manpower was not assigned to combat formations. The day of supply included the full division slice, while the unit of fire seems not to have been used above division level. The ratio of unit of fire to day of supply was lower for heavy weapons, perhaps reflecting the attitude that "the artillery is never in reserve."

Another quite important difference between the unit of fire and the day of supply is that the latter included supplies other than ammunition, such as rations, fuel, and other requirements. Each man required about 45 pounds (20 kg) of supplies per day. These were broken down into five categories for planning purposes by the U.S. Army: 6.22 pounds (2.82 kg) of rations and other consumables (Class I), 3.11 pounds (1.41 kg) of clothing, replacement vehicles, and other general supplies (Class II), 10.67 pounds (4.84 kg) of fuel and lubricants (Class III), 15.46 pounds (7.0 kg) of medical, motor maintenance, quartermaster, construction, and other miscellaneous supplies (Class IV), and 9.58 pounds (4.35 kg) of ammunition (Class V). The figures for fuel (Class III) only accounted for about 10% of fuel usage; the remainder was shipped in bulk tankers and was calculated separately.

There were some interesting differences between theaters. The numbers given above were worldwide averages. The Pacific Ocean Areas adopted the following units of fire for the Marianas campaign:

Units of fire for the Marianas campaign, June 1944

The numbers are significantly smaller than in the previous table. This does not necessarily mean that the Marines were less profligate with ammunition. The unit of fire was never more than a crude approximation of ammunition expenditure in a day of heavy combat, and the Army manual quoted at the start of this article suggests the Army was already trying to divorce the concept of the unit of fire from any notion of a specified time period.

The Japanese Kaisenbun

The Japanese used the Kaisenbun or division-battle for planning purposes. This was the amount of ammunition a division would consume in four months of fighting, assuming twenty days of active combat per month. It was based on experience in the Russo-Japanese War and in China and represented a half to a third of what was required against American divisions.

Japanese Army Kaisenbun ("Division Battle")

The definition of the Kaisenbun more closely resembled the day of supply than the unit of fire. The last column is a calculation of the Japanese day of supply based on the Kaisenbun. This figure should be used with caution, since the assumptions on which it is based are a bit different from those underlying the U.S. day of supply. However, it gives some feel for ammunition expenditure by Japanese Army formations.

A Japanese infantryman typically went into battle carrying 15 kg (33 lbs) of rations, which was expected to last for fifteen days; his rifle and 60 rounds of ammunition; and two hand grenades, along with his helmet, canteen, and other miscellaneous equipment.

Logistics Modeling

The unit of fire is a reasonable baseline from which to model ammunition expenditure during active combat operations. A sophisticated combat simulation will adjust this baseline for different kinds of combat and will adjust combat effectiveness to account for ammunition shortages or abundance. (Troops with plentiful ammunition may be expected to find more occasions to use it.)

The day of supply is a better measure of consumption of non-ammunition items, whose consumption is much less variable. It is a reasonable baseline for the maintenance level of supplies necessary to merely keep a unit up to strength when in contact with the enemy but not engaging in active combat operations.

References
Army Field Manual 9-6 (1944-6-15; accessed 2010-3-20)

Collie and Marutani (2009)

Crowl (1959; accessed 2013-1-16)

Frank (1999)

Garand and Strowbridge (1971; accessed 2013-11-18)

Leighton and Coakley (1955)

Sledge (1981)

The Pacific War Online Encyclopedia © 2008-2011, 2013 by Kent G. Budge. Index

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