Find a Security Clearance Job!


Chapter 4


The achieved muzzle velocity is the result of forces acting on the projectile. To obtain accurate artillery fire, the performance of the weapon projectile family-propellant type-charge combination must be known. If it is not known, the result can be reduced effects on the target or friendly casualties (for example, danger close, final protective fire [FPF], converged sheafs, and so on). Firing tables give standard muzzle velocities for a standard weapon firing standard ammunition under standard conditions. However, muzzle velocities achieved in actual firing may differ from the standard muzzle velocities because of variations in the manufacture of the weapon and ammunition, wear in the weapon tube, projectile weight, propellant temperature, propellant lot efficiency, or a combination of these factors. The M90 velocimeter enables a firing unit to continually update muzzle velocity data. This chapter describes muzzle velocity management with the M90 velocimeter.

4-1. Muzzle Velocity Terms

The following terms are associated with muzzle velocity management.

a. Muzzle velocity--the velocity achieved by a projectile as it leaves the muzzle of the weapon (measured in meters per second).

b. Standard muzzle velocity--An established muzzle velocity used for comparison. It is dependent upon the weapon system, propellant type, charge, and projectile. It is also referred to as reference muzzle velocity.

c. Muzzle velocity variation--the change in muzzle velocity of a weapon (expressed in meters per second) from the standard muzzle velocity.

d. Projectile family--a group of projectiles that have exact or very similar ballistic characteristics. Projectile types within the family are identified by model number.

e. Propellant type--the nomenclature of the propellant used for a particular charge.

f. Charge group--the charges within the propellant type associated with a projectile family, within which MVVs can be transferred. (See Table 4-1.) This has been referred to as propellant model or powder model in the past and in other references. In separate-loading ammunition (155 mm) these terms are synonymous, but in 105-mm ammunition, three charge groups are within a propellant type.

g. Preferred charges--the charges preferred for measuring and transferring muzzle velocities. These charges produce consistent predictable muzzle velocities. The MVVs they produce should not vary more than 1.5 meters per second for the same charge or other charges of the same charge group. Therefore, the MVV determined for one charge of a propellant type will be similar (1.5 m/s) to another charge of the same propellant type and lot. Preferred charges are identified in Table 4-1.

NOTE: The principle of MVVs not varying by more than 1.5 m/s generally holds true within preferred subsonic charges of a propellant type. However, the convenience gained by this assumption more than offsets losses in accuracy, and it is sufficiently valid to allow for accurate massing.

h. Restricted charges--those charges within a charge group to which it is not preferred to transfer measured MVVs or for which it is not authorized to fire (is based on the weapon TM). The performance of a restricted charge is not indicative of the performance of other charges within the charge group.

i. Adjacent charge--charges within a charge group which are 1 charge increment greater or less than the charge calibrated. Used in the conduct of a calibration and subsequent lot inference techniques.

j. Propellant lot--a group of propellants made by the same manufacturer at the same location with the same ingredients.

k. Calibration--measuring the muzzle velocity of a weapon and then performing a comparison between the muzzle velocity achieved by a given piece and the accepted standard. There are two types of calibration--absolute and comparative.

(1) In absolute calibration, the weapon muzzle velocity is compared to the firing table reference muzzle velocity.

(2) In a comparative calibration, the achieved muzzle velocities of two weapons are compared.

l. M90 Readout average--the average MV measured by the M90 which has not been corrected to standard projectile weight and standard propellant temperature.

m. Calibrated muzzle velocity--an M90 readout average that has been corrected to standard projectile weight and propellant temperature.

n. Historical muzzle velocity--a calibrated muzzle velocity which has been recorded in a muzzle velocity logbook.

o. Inferred calibration--the MV of a weapon is determined through mathematical procedures by using data from a first lot calibration (baseline data) and the relative efficiency of a second lot of propellant.

p. Erosion--the wear in a howitzer tube that is the result of firing rounds. It is measured from a pullover gauge reading, which is described in inches, or estimated by computing the equivalent full charges (EFCs) for erosion. This is determined by multiplying the number of rounds fired with a given charge and the number of EFCs per round for that charge and projectile.

q. Shooting strength--the change in the achieved muzzle velocity of a howitzer over time caused by erosion, which is a function of erosion and projectile family ballistics.

r. Ammunition efficiency--the change in velocity which is the sum of the projectile efficiency and propellant efficiency.

s. Projectile efficiency--known deviations from the standard for a particular projectile which effect the achieved velocity. For example, a high-explosive (HE) M107 projectile which weighs 3, 93.9 pounds, vice the standard 4, 95.0 pounds, would have a predictable change in velocity, depending on the charge fired.

t. Propellant efficiency--known deviations from the standard for a particular propellant which effects the velocity of the projectile. For example, a lot of M3A1 propellant may perform differently than the standard for that propellant type but is still acceptable for firing.

NOTE: Refer to ST 6-40-16 for information on the charge group and preferred charges for the 8-inch (203-mm) howitzer.

4-2. Calibration

Three techniques can be used to determine calibration data. The accuracy and complexity of these different techniques varies greatly. Each of the techniques must be understood and applied correctly to the tactical situation. The following order of preference can be used as a guideline. The techniques are listed in order of decreasing preference.

  • M90 chronograph calibration or baseline calibration.
  • Subsequent lot inferred calibration.
  • Predictive muzzle velocity techniques.

a. M90 Chronograph Calibration.

(1) Determine calibration data. The howitzer section installs the M90 velocimeter and records the administrative (admin) data at the top of the M90 Velocimeter Work Sheet (DA Form 4982-1-R). The M90 readout values are recorded in the center portion of the form. Normally, data from six usable rounds, all preferably fired within 20 minutes, are used to ensure the most accurate calibration data. These six rounds can be from any fire mission conducted by the firing unit. Specially conducted calibration missions are not required. If the howitzer tube is cold (that is, has not been engaged in firing or in low air temperatures) the firing of warm-up rounds is recommended. Fewer than six rounds can be used. In these situations, the calibration validity is reduced in the same way that the validity of a registration is reduced when the number of rounds fired is less than normal. In these situations, refer to Chapter 10, Table 10-1 for validity information and the effect of reduced rounds on the calibration data. Powder temperature differences between rounds decrease the validity of the calibration. To reduce powder temperature changes from round to round, use proper propellant handling and storing procedures in the firing unit and fire all rounds measured for a calibration within a 20-minute period. Follow these procedures in the calibration of all weapons. When the admin data and the M90 velocimeter readout data are entered on DA Form 4982-1-R for all weapons, the form is given to the fire direction center.

(2) Determine M90 readout average. The FDO inspects the readout values for all rounds and deletes any invalid readout values, those exceeding the readout average by 3.0 m/s. This 3.0 m/s approximates 4 PER in the target area for the given charge. The FDC personnel then determine the new readout average for the usable rounds by adding all usable readout values and dividing the sum by the number of usable rounds. This value includes the effects of nonstandard propellant temperature and projectile weight.

(3) Correct to standard. The M90 velocimeter readout average is not used in its original form because it includes the effects of projectile weight and propellant temperature on the muzzle velocity. The MV can be used when the corrections for projectile weight and propellant temperature are applied by extracting the value from the appropriate table in the MVCT M90-2 manual and applying that value to the readout average. The correction tables contain data to correct the readout average to what it would have been if the reading had been determined with a standard square-weight projectile and a standard propellant temperature of 70 F. Enter MVCT M90-2 for the appropriate weapon system and projectile family. Locate the page containing the table for the same charge fired in the calibration. Enter the table with the average propellant temperature and the weight of the projectile fired. Interpolate the value to correct the readout average to standard, and apply that value to the readout average. The result is the calibrated muzzle velocity for the weapon.

(4) Complete HE M90 velocimeter worksheet. Once the velocity of the rounds fired has been determined, FDC personnel are responsible for verifying and completing the DA Form 4982-1-R. This will include the steps in Table 4-2. A completed DA Form 4982-1-R is shown in Figure 4-1.

(5) Complete the Muzzle Velocity Record (DA FORM 4982-R).

(a) DA Form 4982-R is the record of a calibration kept in the battery or platoon muzzle velocity log book. The top part of the form (FIRST-LOT CALIBRATION) is used to determine the weapon MVV for a specific charge, when corrected to standard. For future reference, place the completed muzzle velocity record into the unit muzzle velocity logbook under the appropriate weapon projectile family-propellant type-charge group. Ensure this information is given to the platoon leader or XO for entry on DA Form 2408-4 (Weapon Record Data) or NAVMC 10558 (Weapon Record Book, Part I) and 10558A (Weapon Record Book, Part II) for the weapon.

(b) The determined MVV is used in the solution of concurrent and subsequent met techniques and terrain gun position corrections. The lower part of the form (SECOND-LOT CALIBRATION AND SECOND-LOT INFERENCE) is used to infer muzzle velocity data for a second lot of propellant and/or ammunition.

(c) Table 4-3 provides the steps for completing DA Form 4982-R, and Figure 4-2 shows the form completed through the first nine steps.

b. Subsequent Lot Inferred Calibration.

(1) Inferred subsequent lot calibration techniques allow a firing unit to quickly update muzzle velocity information for a given projectile family-propellant type combination, when firing a new lot of propellant. Subsequent lot calibration is used to isolate the difference in efficiency between two propellant lots for one howitzer firing the same projectile family. This difference is applied to the first lot calibration data for the other howitzers to determine calibration data for the second lot. This technique can be used when the situation does not permit the calibration of the new lot with all guns.

(2) To accomplish this technique, the following requirements must be met:

  • Calibration of the first lot must be completed for the entire unit.
  • Calibration of a second lot must be completed for one gun.

(3) A calibration should be completed with all howitzers as soon as the situation allows. Table 4-4 provides the steps for conducting a subsequent lot calibration. Figure 4-3 shows DA Form 4982-1-R completed for a second-lot inferred calibration. Figure 4-4 shows DA Form 4982-R completed for a second-lot inferred calibration.

c. Predictive Muzzle Velocity Technique. While it is not practical to predict (within 0.1 m/s) the velocity of every round, it is possible to approximate velocities to within 1 or 2 m/s with current available information. This may be useful when calibration is not possible, when updating calibration data, or when trying to increase the accuracy of inferred MV techniques.

(1) When calibration is not possible, the shooting strength of the howitzer can be used as the MVV. While this may be enough when no other data are available, it is important to understand that an MVV consists of more than just shooting strength. An equation can be created for determining an MVV by using its basic parts. (See Figure 4-5.)

(2) If all three elements are known, it is possible to determine a value for MVV. It is neither practical nor necessary to quantify round-to-round variation. This element is usually small and subject to rapid change. Projectile efficiency, as a part of ammunition efficiency, is accounted for in solving the concurrent and subsequent met techniques. Therefore, if the round-to-round variation and the projectile efficiency are eliminated from the equation, the howitzer shooting strength and the propellant efficiency of the propellant lot to be fired can approximate the MVV. (See Figure 4-6.)

(3) If calibration is not possible, adding the propellant efficiency to the shooting strength will result in a more accurate MVV for determining firing data than if the shooting strength is considered alone. This MVV can be used as the MVV for manual fire missions. Each howitzer has a value for shooting strength for each projectile family. Also, the value of propellant efficiency applies to any projectile family with which the propellant lot is fired.

4-3. Estimating Shooting Strength

a. There may be times when calibration is not possible. If the M90 is not available or there is not time to conduct a calibration, it may be necessary to determine the shooting strength of the howitzer by other means. The shooting strength of a howitzer can be determined by using pullover gauge readings and/or erosion EFC service round effects with the appropriate TFT for the weapon-projectile combination to be fired. (See Table 4-5.) DA Form 2408-4 provides the information to determine the shooting strength of each howitzer. (See Figure 4-7.)

b. The number of EFCs is determined by multiplying the number of rounds fired for a specific projectile and propellant by the equivalent erosion effect in decimals for the charge fired listed in the introduction of the TFT. Different projectile families have different TFTs and consequently different values for equivalent erosion effect in decimals. Pullover gauge readings can be determined regularly by the maintenance section in conjunction with borescoping the howitzer. The most accurate technique is to combine the pullover gauge reading and the erosion EFCs fired after the pullover gauge reading to determine an expected loss in muzzle velocity. The most recent pullover gauge reading or total erosion EFCs may be used to determine the approximate loss in muzzle velocity. Table 4-6 provides the steps for determining the pullover gauge reading.

4-4. Updating MVV Data

Once determined, the calibration data represent the best indicator of the expected MVV. But the MVV is not valid forever since the howitzer shooting strength changes as more rounds are fired. Calibration data can be made indefinitely valid if the shooting strength is continually updated. If the shooting strength of the howitzer is determined at the time of calibration, the changes in shooting strength can be added to the calibrated MVV over time. The current shooting strength can be determined from the Approximate Losses in Muzzle Velocity table in the TFT introduction on the basis of the pullover gauge readings and/or EFCs. The change in shooting strength is determined by subtracting the shooting strength at the time of calibration from the current shooting strength. This difference is then applied to the calibrated MVV to determine a current or updated MVV.

4-5. Other Applications

a. The possible applications of the basic equation, Figure 4-6, provide enough flexibility to determine any of the three parts of the equation (MVV, shooting strength, and propellant efficiency). If any two parts are known, the third can be determined. For example, if a replacement howitzer is received without its DA Form 2408-4, its shooting strength would be unknown. However, if it was calibrated and the propellant efficiency of the calibrated propellant lot is known, the shooting strength can be approximated (a modification of the formula found in Figure 4-6). This approximated shooting strength could then be applied by using the techniques described in paragraphs 4-2 and 4-3.

b. A useful application of the basic equation is the ability to solve for the propellant efficiency of a given propellant lot. The propellant efficiency could then be used to predict an MVV for a different projectile family. (See the example below.)

4-6. MVV Logbook

Once MV data have been determined, these data are used for numerous techniques. MV data must be recorded on DA Form 4982-R which is then filed in an MV logbook. The MV logbook allows for quick referencing of howitzer performance when firing a particular projectile family-propellant lot-charge combination. The major sections in the MV logbook are for the projectile families. Each one of the sections should be tabbed for each authorized propellant type-charge group for the projectile family.

4-7. Frequency of Calibration

Ideally, calibration occurs continuously. If that is impractical or impossible, the following methods identify when to consider calibrating.

a. Initial Receipt or Retubing. All new pieces of a given caliber and model will not necessarily develop the same muzzle velocity because of the tolerances that are allowed in the size of the powder chamber and in the dimensions of the bore. Therefore, pieces should be calibrated as soon as possible after receipt or when retubed. Muzzle velocities should be recorded on DA Form 2408-4.

b. Change in Propellant Lot. Calibration should be conducted as soon as possible after an uncalibrated propellant lot is received.

c. New Projectile Family. Calibration should be conducted if a new projectile; for example, M825 smoke (projectile family DPICM), is received for which there are no previous MV records for that projectile family.

d. Annually. Any piece in service should be recalibrated at least annually. The primary factor contributing to the loss in muzzle velocity for a piece is the number of rounds that have been fired through the tube and the charges used in firing them. Higher charges increase tube wear, which, in turn, tends to decrease muzzle velocity. Guns, because of their higher velocities, tend to display tube wear more quickly than howitzers. If a great deal of firing takes place, recalibration will be needed more often than annually. Methods of determining when recalibration may be needed are outlined below. The following situations assume that firing takes place with a previously calibrated projectile family-propellant lot.

e. Changes in VE. If an accurate record of the changes in VE determined from concurrent met solutions is maintained, it may be used as a guide for determining the need for recalibration. When the velocity loss since the last calibration is equivalent to 2 range probable errors, the need for recalibration is indicated. (An indicator of this is a loss of 1.5 m/s, which generally approximates 2 probable errors in range.)

f. Tube Wear. The extent of tube wear near the beginning of the rifling of the bore indicates the loss in muzzle velocity and the remaining tube life. Precise measurement of the distance between the lands in the bore near the start of the rifling can be made with a pullover gauge. Organizational or direct support maintenance has this gauge and makes the measurement. The wear measurement, when compared with the data in the "wear" table (Approximate Losses in Muzzle Velocity table) in the introduction of each firing table, can be used in estimating the loss in muzzle velocity.

g. EFCs. A change in the number of erosion EFC service rounds as depicted in the weapon record book may also indicate a need for recalibration. (Refer to paragraph 4-3 for more information about EFCs.) The change in erosion EFC rounds compared with data in the Approximate Losses in Muzzle Velocity table (in the introduction of each TFT) that corresponds to a loss of 1.5 m/s in muzzle velocity may indicate a need for recalibration. A loss of 1.5 m/sin MV generally equates to the effects of 2 probable errors in range (2 PER).

4-8. Transferring MVVs

a. Ideally, every charge should be calibrated. However, this may not always be feasible. Therefore, the calibration of a few charges, one within each charge group that results in an MVV applicable to other charges within a charge group, is imperative. For calibration purposes, there are two categories of charges within a charge group. These are preferred charges and restricted charges. The following guidance is established as an order of preference when selecting a charge to calibrate:

(1) If you know the charge you will be firing calibrate that charge.

(2) If the charge you will be shooting is unknown, calibrate the middle charge of the preferred charge group.

b. Calibration data determined should only be applied to a subsequent fire mission when the mission meets the following requirement: It is the same calibrated howitzer firing the same calibrated projectile family-propelant lot combination. Once calibration data are determined for a particular charge, these data can be transferred to other charges in the same lot. The order of preference for transferring is as follows:

(1) Transfer down 1 charge.

(2) Transfer up 1 charge.

NOTE: Shooting strength and ammo efficiency make up the achieved MV. With higher charges, there is more erosion but less variance in ammo efficiency. For lower charges, there is less erosion but more variance in ammo efficiency. Therefore, the general overall effect is less variance when transferring down as opposed to up.

(3) Transfer down 2 charges within the preferred charges.

(4) Transfer up 2 charges within the preferred charges.

(5) Transfer from a preferred charge to a restricted charge.

NOTE: MVVs should not be transferred from a restricted charge to any other charge on the basis of the nature (large round-to-round variances) of restricted charges.

Join the mailing list