Information Sheet Number: 1.7
Excessive use and improper or infrequent maintenance reduce a gun barrel's longevity. A gunnery officer needs to be aware of the conditions that contribute to, and the effects of, gun wear. If confronted with an unsatisfactory barrel condition, the gunnery officer has to recognize the fault and effect the required actions.
(a) SW300-BB-GTP-010 Serviceability of Naval Gun Barrels
(b) NAVSEA 8300/2 Gunwear Star Gauge Report
A. GUNWEAR TERMS
1. Gunwear is caused by erosion and corrosion of the gun barrel.
(1) Rate of removal of metal from interior surface of the gun barrel.
(2) Caused by heat versus time versus pressure build up.
(3) Any bore surface cooling scheme will decrease erosion.
(a) 76mm with the water jacket
(b) Solid coolant (talc-wax) in 5" MK 73 propellant charge
(1) Deterioration and decomposition of metal from the interior surface of naval gun barrel.
(2) Generally caused by neglect or lack of preservation.
(3) The chief contributor to gun wear is the failure to perform proper PMS IAW NAVSEA instructions and PMS cards.
(4) The only approved solution for cleaning naval gun barrels is CLP breakfree.
B. COMMON TERMS AND DEFINITIONS
1. Major parts of the barrel
a. Slide cylinder
(1) High pressure - high wear area
(2) Contains the chamber which is the powder charge receptacle
(3) Also contains the forcing cone
The forcing cone guides the projectile out of the larger chamber into the smaller bore.
(4) Origin of the bore
(a) First place from the breech face to the muzzle where the lands reach their maximum height.
(b) Also the first place in the barrel where the diameter of the barrel reaches the desired dimensions (ie. 3", 5" etc)
(c) A critical dimension used as a reference point for star gauge reading and muzzle velocity calculations.
(5) Chase - low pressure, low wear area
(6) Muzzle - low pressure, high wear area
2. Gun barrel fatigue process
a. Heat checks
(1) Also known as crack initiation.
(2) Small (sometimes invisible to the naked eye) cracks in the surface of the bore.
(3) They can reach an approximate depth of .005" to .025".
(4) May also start to occur after firing approximately 300 rounds.
b. Slow crack growth
(1) Once the heat checks/cracks are formed, these cracks continue to grow slowly under the influence of stress in the gun barrel wall arising from the pressure versus time history during firing.
(2) At this stage the heat check/crack will appear in a checking pattern and will be deeper than .025".
(3) Heat checks/cracks will then connect together to form longitudinal and circumference cracks.
(4) Longitudinal cracks which are long and continuous and reach a length of approximately 2.5" to 3" will result in the condemning of the gun barrel.
(5) Circumference cracks which extend approximately one of third the inside circumference of the gun barrel are justification for condemning the gun barrel.
c. Fast crack fracture
(1) When the crack grows at a very rapid rate (which can reach 5000 ft per sec) a condition known as fast fracture is reached. This condition produces catastrophic failure of the gun barrel structure
d. Gas washes
(1) Also known as flame washes
(2) Generally occur near the origin of the bore
(3) Steel in the barrel physically melts away.
(4) Caused by hot high velocity gases
e. Gas pockets
(1) Concentrated area of gas washes
(2) Melting of the gun barrel interior surface causing imperfections
(3) Gas pockets which obtain a depth of .100" constitute criteria for regunning.
3. Service life criteria
a. Safe fatigue life - a safety restriction intended to prevent structural failure to the gun barrel leading to possible destruction of the gun mount and personnel.
(1) Safe fatigue life estimates are established by NAVSEA.
(2) When at or near safe fatigue life estimates, ship should request regunning.
(3) Type Commander's permission must be granted to exceed safe fatigue life estimate.
b. Erosion life - a performance standard intended to guarantee a certain level of system performance in three areas:
(1) Maximum range
(3) Fuze performance - less setbacks, improper centrifugal force, gases escape around projectile.
c. The gun barrel is removed from service when any area becomes unsatisfactory.
C. PROJECTILE SEATING DISTANCE GAUGE
1. Projectile Seating Distance (PSD) gauge
a. Measure actual distance of projectile seating in the bore
(1) Three single readings
(2) Utilizes 360 degree measurement.
(3) This is a special tool for the 5"/54 gun. Gunnery Officers should ensure this tool is carefully handled and safely stored.
(4) The end result is an increase in barrel life based on erosion life data.
(5) PSD gauge will be used to determine loss of velocity in the range table and Initial Ballistic Corrections (IBC) section of this course.
D. FLEET STAR GAUGE MK 8 MODS 0 AND 1
1. Used to measure gun wear
2. Estimate the velocity loss due to gun wear and to estimate gun life because wear at the origin of the bore and wear through the bore do not progress at the same rate.
3. Service gun life estimates are purposely designed to indicate regunning before performance actually deteriorates to a serious degree.
4. Fleet star gauge report - required annually.
5. Fleet star gauges are not provided to most combatant ships. Tenders and repair activities normally provide star upon request.
6. Point at which star gauge reading are taken:
a. Origin of the bore
b. 1" Forward of the origin of the bore
c. 12" Forward of the origin of the bore
d. 12" Aft of the muzzle
e. 1" Aft of the muzzle
f. At the muzzle
7. Determine and record gun wear percentage on NAVSEA Form 8300/2 - refer to SW300-BB-GTP-010 appendix A
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