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Small Arms Ammunition (Cartridge Case Manufacture (Rimfire (rolled sheets…
Small Arms Ammunition
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Components
Introduction
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Anneal - change metal grain size to restore softness and ductality that is reduced in cold-work; dont at high temperatures
Propellants
Nitrocellulose - cellulose treated with nitric acid in presence of sulfuric acid; common propellant substrate
cellulose used to come from cotton linter; replaced later by synthetic fibers and then by wood fibers
step 1: create nitrocellulose; nitrate radicals take place of some hydrogens in cellulose; sulfuric acid takes up excess water
step 2: incorporate solvent to make nitrocellulose cohesive; for single-base propellants, use alcohol/ether mix (adds no energy); for double-based propellants, use nitroglycerin (adds large amount of nitroglycerin - explosive tendencies controlled by mixing with other materials)
plasticizers - soften the substrate they are added to (the solvents, in this case)
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Morphology
granule - chunk of nitrocellulose; size and shape control rate of energy release; solid granules burn outside in, so as surface area decreases so does energy release; larger granules take longer to release all energy
disk (flake) propellants - burn flat-side inward, so energy released is constant; used in low-pressure, low-volume arms; produced via die extrusion
lamelle - first shape of modern propellants; same burn characteristics as disks; nitrocellulose rolled flat; sheet put through rollers with cutters to make diamond-shaped pieces
cylindrical - larger surface area, medium to slow release; some have perforations to keep surface area more uniform to get constant peak pressure
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aggregate ball - form of ball propellant; granules are aggregates of smaller balls; size varied; not really used today
Die Extrusion
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extrusion of nitrocellulose done with rotating cutters; extrusion makes the long shape (ie. long cylinder) while cutters cut long shape into pieces (ie. smaller cylinder pieces)
change extrusion holes, extrusion rate, and cutter speed to get different shapes (ie. slow extrusion + fast cutters = disk; fast extrusion + slow cutters = cylinder)
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Making Ball Propellants
granules moved to shaper filled with liquid that keeps granules in constant motion rather than falling into liquid tank
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for double-base ball powders, nitroglycerin added after shaping via surface impregnation
granules sorted by size; coatings added before drying; roller pressed if powder is to be flattened ball
Drying
need to remove excess water to prevent aggregation; too little water makes granules crumble, increasing surface area and making propellant burn too quickly and create too high pressure
dry with lot of air in rotating drum at low heat (heat removes needed solvent in single-base and makes double-base brittle)
surface treatments added during drying (ie. antistatic coating from graphite - gives propellant grey color)
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Testing, Grading, and Blending
reference lot - control sample; test new batches of propellant against it; usually first successful lot produced
testing uses bomb calorimeter; burn known mass of substrate inside and measure heat production; if result matches reference lot decently, new batch goes to ballistic testing
load some cartridges with reference lot and some with new batch; test for pressure and velocity; if results match well, new batch approved for shipment
if batch fails to meet either calorimeter or ballistic standards, it may be blended with other powders to adjust energy release rates
Canister and Bulk
canister propellant - sold in small amounts for people who reload cartridges; ammunition manufacturers have tested cartridge, bullet, and propellant combinations to produce safe guidelines
bulk propellant - sold to ammunition manufacturers; manufacturers have equipment to test pressure and determine amount of propellant needed for loading - don't need published guidelines
Projeciles
Casting
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Cons: need to pay attention to lead temperature, mold temperature, rate of pouring, alloy consistency, and rate of cooling, or bullets will not be uniform
Pros: can produce sharp edges and details; can be used to form projectiles that are too hard to cold form;
Swaging
Swagging - die shaped to match profile desired product; lead forced into hole in die to shape it; usually done cold
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after cannelure, bullets are lubricated
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Current Manufacture
Shot pellets manufactured with Bliemeister method; (1) molten lead goes through several screens that are agitated (2) droplets fall short distance into hot water (3) pellets roll down incline, sort by shape (4) nonrounded and rejected shapes remelted
most buckshot made with lead wire; steps: (1) wire fed into counter-rotating wheels (2) hemispheres milled into rim of each wheel (3)hemispheres of each wheel meet as wheel turns (4) balls formed where cavities meet, excess lead flattened (5) flattened strip tumbled to remove excess led and remove pellets
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Lead-Free Shot Pellets
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soft steel first substitute; molten steel ejected at high speed and pressure into water spray; steel hardens to spheres; sort by shape
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various mixtures of tungsten, nickel, bismuth, etc. to increase density; some mixtures sintered
Jacketed Bullets
replaced plain lead around when smokeless repellents replaced black powder; higher temperature and pressure would deform unjacketed lead
copper alloys standard for jackets; modern alloys used: gilding metal, commercial bronze; alloy choice depends on desired jacket thickness - stronger alloy for thinner jacket
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Cup and Draw Operations
shallow cup formed from sheet metal and cupping press; die and punch blanks disk from sheet metal and forms it to shallow cup
some jackets that are not much taller than they are wide can be made directly from pres; when bullet is much longer than it is wide, cup must undergo draw operations
drawing - stretch under controlled conditions, reducing diameter; control from die and punch set that stays in contact with jacket walls so stress is applied equally at all points
drawing allows controlled expansion - elongates metal parallel to length of jacket, creating natural parting lines so jacket peels back upon impact
advantages of controlled expansion: bullet integrity maintained after impact, bullet remains in target, energy dissipates more quickly on target
Bullet Features
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sharp-pointed full metal jackets formed with coining die; die traps jacket between punch and die, forcing material to sharp point
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Centerfire Ammunition
Case Mouth Sealant
sealant added to inside of case mouth before other loading operations if cartridge is to be sealed (prevents moisture)
black asphaltic tar preferred; applied wet, dried, and cured
Priming
cases whose flashhole is not opened during manufacture get off-line priming via pierce-and-prime machine
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annular crimping or staking for military primers in semiautomatic and automatic weapons; prevents pressure from forcing primer backward
annular crimping - alters edges of primer pocket; get rim of brass overhanging primer to make secure lock
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Propellant Charging
high-speed continuous loading machines used; low-volume cartridges still use slower interrupted sequence machines
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Bullet Seating
high-speed hazards - inverted projectile, misalignment
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Shotshell Assembly
Priming and Loading
priming done after case manufacture - separates primers from propellant charging operations - less hazardous
primer fed into position and seated with punch; designed to push against battery cup and leave primer cup alone; reduces misfire
loading - selected propellant selected for shotshells; wadding material inserted; appropriate projectile selected; crimp end of case to keep stuff inside
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Wadding
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inspection - sensor verifies that wad is present, in alignment, and undamaged
projectile loading - usually lead or nontoxic pellets; measured by volume; guide tube to minimize spills; nontoxic shots may need special wad and heavier shot cup - protects gun barrel
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Crimping
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crimper - die with concentric punch that fits outside of case; flat punch pushes folds made by starter, making the fold flat
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can be sealed with wax or other thermal sealant to resist moisture, or the folds may be welded together with heat
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Testing
same testing as rimfire with exception of accuracy testing; random samples that use pellets tested for patterning
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Primer Manufacture
Rimfire Priming
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primer mixture of lead styphnate, antimony sulfide, barium nitrate, and other chemicals; creates heat and gas when struck sharply; placed in hollow rim; binders hold it in place; may have frictionator (helps ignition)
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steps: (1) wet mix forced into nonsparking metal charge plate through small holes (2) plate placed over tray of rimfire cases (3) third plate with multiple pints presses through charge plate (4) primer disks fall from hole plate into cases
downward and centrifugal force applied during spinning to make sure mix is distributed in rim uniformly; vacant spots can cause misfire
Testing
sample selected from each lot and put in machine with vertical cartridge chamber under steel ball of known mass
ball dropped on cases from several heights; at each height, the percentage of cases that fire are noted
Centerfire
primer, cup, anvil, priming chemicals; sometimes foil paper
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priming mix blended at explosive chemical facility; kept wet for safety; cups and anvils brought to chemical vacility
wet primer forced into charge plate; wet pellets ejected to primer cups; press forms pellet to fit bottom of cup; foil may be added on top to prevent pellet from sticking to compaction pins
anvils added to charged cups and pressed in with part above edge of cup; drop of nitrate added for sealant
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Shotshell
battery cup - supports and strengthens primer; copper, brass, or copper-plated steel
anvil larger than rifle or handgun; flat plate perpendicular to cup; edge rests against inside of battery cup
primer cups pressed into battery cup, which already contains anvil
battery cup hollow; avoid fine propellant entering cup - can add pressure inside cup at firing that damages bolt or breechface
Ballistic Testing
Intro
tested against company standards nd standards of Sporting Arms and Ammunition Manufacturers' Institute Inc. (SAAMI)
SAAMI guidelines voluntary, but recommended to avoid unsafe and incompatible products
tests: pressure, velocity, function, accuracy, customized specifications, cosmetic evaluation
Pressure
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crusher testing - mostly obsolete; barrel placed over cartridge, piston touches case, crusher sits on top of piston; cartridge fires and pressure pierces case and drives piston into crusher; measure length crusher moves
piezoelectric transducer testing - most used method; cartridge fired in barrel with sensors that detect pressure; transducer in barrel creates electric signal from compression force
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Assembly
Rimfire
too little propellant: bullet may stick in barrel; too much propellant may cause case to fail, injuring shooter or damaging firearm
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Bullet Seating
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conveyor loading - die falls on case mouth, bullet is fed into die, and seating punch presses bullet into case;
plate loading - one plate holds bullet and other holds case, plates are aligned, and press pushes bullet into case
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Source: Savage K, Freed G. Firearm Examiner Training. National Forensic Science Technology Center. [accessed 2019 Jan 25].