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Modern Firearms Manufacture (Rifled Barrels (Hammer Forged (reusable…
Modern Firearms Manufacture
Metal-Forming Operations
Introduction
Gross Forming - reduce raw stock to intermediate of desired shape
fine forming - renders intermediate to final form
Casting
historically, sand molds used - molten metal poured into mold and allowed to set; mold must be able to tolerate temperature of metal
wooden pattern of the desired positive made and packed in sand in two parts; halves called the cope and the drag
halves opened and wooden positive is removed, leaving behind the desired negative
halves fit back together and molten metal poured into sprue hole in mold
once metal has hardened, the halves are separated and the product is removed; the mold can then be used for further cases
alternatively to wooden positive, wax may be used and melted out of the mold
multiple parts may be cast at once - connected by runners to let molten metal fill all from one hole; parts are broken apart at runners; may need to finish rough surface
alloys with iron tend to be brittle when cast due to slow cooling - cast iron gun parts not really used anymore
Investment Casting
uses sacrificial wax in master mold
can make precision parts from strong alloys - reduced finishing
precision wax form attaches parts via runners
assembly dipped in ceramic slurry repeatedly to build shell around wax (like candle dipping)
fine sand may be introduced as layer becomes thicker to strengthen shell; the early ceramic-only layers maintain delicate detail because ceramic finer than sand
after shell is built, assembly is heated to melt and remove wax
remaining mold heated in furnace - burn off rest of wax, fuse shell
mold preheated to slow cooling time and allow details to set; molten metal poured and mold is allowed to cool
mold is broken away
parts are broken apart at runners and finished
pretty much replaced sand casting
Die Casting
permanent mold for weaker, nonferrous metal (usually zinc alloys)
advantages: reusable mold, zinc easier to melt (less energy), product parts easy to machine
disadvantages: difficult to produce precise tolerances; can get voids or cracks, product weak and wears easily; untreated zinc alloys corrode easily
handguns with cheap zinc frames usually chrome-plated to cover surface roughness and flaws - adds to cost significantly
Materials and Methods
increased usage of plastics for non-stressed parts
injection molding used for precision plastic parts
molten plastic forced into preheated mold
channels in mold flushed with cold water while plastic is still in mold to cool plastic surface quickly
dimension-critical parts left to cool in mold to prevent warping
Forging
forging - hammer metal into a shape; one of oldest metalwork arts; hand forging - traditional pounding hot metal on anvil
early gun barrels hand-forged around mandrel (rod of known diameter); worked two plates of hot steel around mandrel and fused edges
can use hot or cold metal, but glowing hot metal easier to shape and less likely to break; cold sometimes used on smaller parts (swagging)
metal molecules also get compressed, strengthening it
machine forging - dies (mold edges) carry negative impression of part and press into material simultaneously; consistent size and shape
hammer forging (drop forging) - one die stationary on anvil, other die attached to drop hammer; machine lifts ram and drops it on stationary die with high force; hot metal placed on stationary die conforms to die shape; can be open or closed die forging; closed used when excess metal flow needs to be controlled
press forging - force applied slowly; good control of material
benefits: products strong and durable; fewer variations in grain structure because substrate heated, not melted; alloys don't separate from melting; improved durability from compression, can design areas that require special stress-strain performance because natural flow lines controlled
used for firearm parts with high demand (frame, bolt, cylinder)
Stamping
flat metal sheets put on press with dies; press activates and shapes part from die
used for thin parts (ie. magazine, trigger guard)
fine blanking - subset of stamping; makes small parts (triggers, hammers); limited by material thickness, but identical parts can be stacked to make thicker part
common in military, law enforcement, and commercial firearms
Powdered Metal Technology
uses powdered metal instead of molten; heated until particles melt slightly and fuse together
very precise but not as strong - used for low-load parts
Extrusion
used for parts with uniform cross sections; long strips cut to final dimensions later; used in low-loading parts with nonferrous alloys
bar of substrate pressed through negative die
Fine-Forming Operations
Introduction
prepare intermediate part for finishing touches
mostly machining - use moving tool to remove metal from intermediate to meet final dimensions and form; tool must be harder than part it is shaping; need to control frictional heat
Drilling
produces a hole
drill bit - tool with sharp working edge
chips - residue from machining; must be cleared to prevent tool from binding; spiral drill shaft removes chips from hole
hole through barrel of handgun or rifle can mark fired bullet - foundation of unique characteristics of a gun barrel - makes forensic identification possible;
Turning
rotate object about axis and bear cutting tool
used for parts with precise outside diameter with lathe (ie. barrel)
can also be used to form flat faces on ends of a bar
bolt faces often turned - makes random marks that transfer to cartridge case as breechblock mark
Milling
part held against precision block and moved against rotating cutting tool
block moves length, width, and height; advanced ones move 3 other directions (roll, pitch, and yaw)
can do complex contours; can use a custom cutter to produce a specific shape
can position multiple cuts separated by a particular angle
flexibility and uniformity
parts that are milled and can leave identifiable marks on casings: breech face, extractor, ejector, ejector port, noncircular firing pins, cartridge guides)
Grinding
spin hard, abrasive wheel against metal at high speed
grit size can make part surface rough or smooth
Broaching
multiple cutters on same axis; remove excess metal progressively; each successive cutter is slightly larger than the last
can be used to make barrel grooves (rifling)
Computer Numeric Control Machining
combines multiple processes into one machine; reduces required space but with higher output
good for mass production of identical parts; minimal operator assistance so one person can man several machines
computer codes the design of the finished product and has machine perform particular processes on substrate
operator may need to remove finished product and reload next substrate and restart machine; some automatic machines can do this themselves
engineer designs part in 3D with drafting software; data sent to CNC center and software converts drawing to Cartesian coordinates, determines necessary tools, and decides order of operations
cheaper, better quality (reduced human error
Electrical Discharge Machining
electrical path between electrode and work surface burns away material for a particular shape
mainly used for complicated holes and cavities
electrode formed to desired shape and inserted into pilot hole
current applied, creating arc from electrode to work material
metal erodes in shape dictated by electrode
Hand Fitting
filing, stoning, and pressure fitting parts
can show up on breechface markings from revolvers
filing used to adjust gap between cylinder and breechface; products never filed same way twice - creates distinct microscopic marks
Rifled Barrels
Introduction
Rifled Barrels
barrel begins as solid steel bar; type of material has no large impact on identification
firearm companies may make own barrels or buy partially completed ones; modern manufacturing tends to buy parts to reduce investment in large amount of equipment
Reaming
smooths surface, corrects small errors in straightness; creates final desired diameter
bore diameter = minor diameter
groove diameter = major diameter
reduces roughness left by drilled holes
long cutting tool; edges on sides; diameter same as bore diameter
may still leave markings on a bullet for identification
Lapping
polishes barrel interior after rifling to remove microscopic projections; used in high-grade barrels or barrels designed for soft lead bullets
put abrasive paste on soft lead cylinder and pass it through barrel several times; finer past produces smoother surfaces
Deep Hole Drilling
starting steel bar is straight - helps boring and rifling alignment and prevent deformation from heating
cut hole straight down length of barrel; may be smaller than bullet fired for cut rifling; larger hole for forged rifling
requires lot of support to make sure hole is straight; large hold can be drilled faster because drill shaft stiffer (prevents warping)
chips from drilling make marks on inside of barrel before they are removed with lubricants - useful for identification
drills generally yield hole slightly smaller than necessary diameter - fixed by reaming
Straightening
aim barrel at pattern on wall; pattern designed so its reflection on interior of barrel shows strightness of barrel
if pattern suggests barrel is not straight, barrel is placed in hand press and pressed to point barrel to improve straightness
manual - requires skilled operator; increases cost - better to use good machinery the first time
Rifling
4 ways: cut rifling (single-point or broached), button-swagged rifling, forged rifling, electron discharged machining (EDM)
electrochemical rifling - wet-etch interior of barrel under electric current; interior metal is eaten or dissolved away to make grooves
electrode has metal strips in shape of rifling; electrode placed in barrel; assembly placed in salt solution; electrical current applied and electrode moved down barrel and twisted, metal is removed by electrolysis
fast; no consumable tooling
unique characteristics - identification
Cut Rifling
Single Pioint Rifling
grooves cut one at a time; oldest method; still used in some barrels today
cutter head - small pies of hardened alloy supported by wedge-shaped base; cutting surface extends from top - size equal to barrel groove width
tool holder - cylinder slightly smaller than barrel; holds cutter unit; channels to allow for cutting oils to flush chips during use; other end attached to rifling machine
machine has clamp to allow precise rotation of barrel for cutting and rotating mechanism to turn cutter holder as it moves to make spiral grooves
mount barrel in machine, set direction and twisting rate
machine moves cutter through barrel to produce groove; barrel is rotated and another groove is cut; angle between grooves equal
after all grooves have been cut once, cutter holder increases its cutting depth and does another pass; this continues until desired depth is reached
slower and more costly than other methods, but still competitive
advantages: lowest part replacement costs; flexible characteristics for rifling; low stress on barrel; can change rifling characteristics quickly, easy to adapt to steel alloy changes
preferred rifling method for specialized barrels, not so much mass-produced ones
Broached
rod with multiple cutting surfaces that get progressively larger
lot of dimensional control
on disk, smaller diameter between cutting surfaces to allow room for coolant and chip removal
barrel placed in clamp that rotates barrel during broaching to create the twist
unique marks occur due to chips prior to their removal
Button Swaged
cold-formed rifling; metal moved, not removed
rifling button - double tapered metal plug; must be harder than barrel material; must be made of heat-treated steel or tungsten carbide (preferred - high density makes it more durable)
higher areas of button press grooves into barrel; low areas allow for coolant; button is negative of the desired rifling
minimal chip production
machine pulls button through barrel to produce rifling
durable; low cost; high production rates
best for small-diameter firearms
no significant unique marks made unless button has damage
Hammer Forged
reusable mandrel that is negative of entire barrel length
starting barrel should be slightly larger than intended maximum diameter of finished barrel
barrel length should be shorter than desired finished barrel
barrel clamped; mandrel inserted; barrel moved at constant speed; hammers strike rotating barrel to bend steel to contract against internal mandrel
elongates barrel; outer surface has peening marks from hammer hits; steel elastic - moves away from mandrel following release of force
equal compression affected by rate of barrel feeding into hammers, rotation rate, hammer hit frequency
integral chamber may be formed during foraging using a full-length mandrel
identification marks mainly come from prior drill and reamer marks and any chambering or crowning process marks
Electrical Discharge Machining
use more for making dies, especially tungsten carbide dies because tungsten carbide is harder than steel (replaced diamond past in shaping die cavity - diamond paste now needed only for final polishing)
Contouring/Profiling
barrel must be same thickness at every point along its length - if not, stress and vibration during firing will be asymetrical
profiling commonly added by lathe-turning following rifling
muzzle crowning - grind away interior of barrel at end of muzzle so rifling doesn't go all the way to the edge; usually uses steel lap with grinding paste; can contribute to unique marks
Chambering and Throating
chamber - firearms that use cartridges need back of barrel to be enlarged to accept cartridge; use chamber reamer to carve out
throat - where chamber meets barrel; must be tapered carefully to reduce force on bullet; throat reamer integrated into chamber reamer for mass production; special throating reamer may be used for special cases
Barrels for Muzzleloading Arms - Shotgun
not point target firearms - prioritize pellet pattern over accuracy
thin barrels required to reduce weight of larger barrel; mostly hammer forged - include chamber shape on mandrel; lack rifling
sport shotguns choke-bored (tapered at end) - good for hammer forging, controls pellet spray; tighter constriction for longer range
hammer forging lets choke to be formed simply by having end of mandrel be slightly smaller
modern shotguns use threaded interchangeable chokes
modern rifled shotguns use shallow polygonal rifling made by mandrel pattern; use small projectiles with sabot, so just need enough rifling to grip and spin sabot
Parts Fabrication
High-Stress Load Parts
Frames or Receivers
frame (aka receiver) - basic component of firearm; contains firing and breech mechanisms
may be two parts in some military firearms and similar commercial firearms; two parts hinged - upper receiver tips up for cleaning and disassembly
Ejection Port and Feed Ramps
can form identifiable marks, especially ejection port marks
index points from final forming operations on frames, receivers, and other large parts
Slides
upper unit on most semiautomatic handguns; usually includes bolt or breechblock
closed for firing via spring pressure or lock
bolt or breechblock presses against end of new cartridge case and locks barrel before firing moves cartridges in and out of chamber; can leave identifiable microscopic markings
breech face - where breechblock contacts cartridge head during firing
Cylinders
strong alloy for powerful handguns to contain firing pressure
need same angular distance between chambers to consistently move bullet to barrel; improved by modern computers
indexing surfaces must be durable and precise
Low-Stress Load Parts
Lock Parts
hammer and trigger exposed to shock and recoil
can be fine blanked when aesthetic doesn't matter (ie. military arms and cheap commercial arms) - get uniform thickness
for non0uniform thickness, forging used to be standard; modern casting and powdered metal replacing forging
small parts (sear, safety, disconnector) require precision - CNC milling still common, but investment casting and PMT are starting to replace it
firing pins symmetrically rod shaped; made with lathes; magazine loaded with rods feeding into cutting unit; rod is profiled to set pattern; cutter can make coarse marks (subclass characteristics) that repeat between pins - can be used to exclude but not match
Repeating Mechanism Components
varied form, function, and method of manufacturing
pump-action and semiautomatic shotguns have shell latches that can leave identifiable marks on shells
Small Pins, Screws, and Springs
firearm manufacturers buy small parts from others
Detachable Magazines
box magazines made from fine blanked and folded sheet steel; follower and springs added to finish
moving towards polymer magazines; some have metal feed lips cast into main body or attached at assembly
can leave marks on cartridge sides, especially with metal feed lips, but those can carry over between magazines
Cosmetic Finish
Introduction
external finish improves appearance and protects against environmental damage
may be applied before or after fitting and assembly; usually before assembly
before civil war, standard finish for military firearms was polished bare metal; removed tool marks, but did not protect or color metal; resulted in slow oxidation of firearm
U.S. military adopted commercial finishes in 1873, most commonly blued steel
Bluing
protective coating for steel parts
chemically oxidizes metal to prevent corrosion
can turn out either blue or blue-black
Electroplating
protective coating for different types of metal, most commonly used on chrome or nickle plating to give bright finish
results in thin metal coating over exterior of metal part
Anodizing
protective coat for aluminum and titanium
controlled oxidizing using electrical current
can produce different colors, but black most common
Other Finishes: Parkerizing (phosphate coatings on steel) case hardening, and paints
Source: Savage K, Freed G. Firearm Examiner Training. National Forensic Science Technology Center. [accessed 2019 Jan 10].
Assembly
Subassembly
complete components consisting of multiple pieces
parts requiring: hammers, triggers, cylinders, bolts, slides
usually needed for major components with complex functions
Final Assembly
creates final functional firearm
work required depends on firearm design
drop-in fit - fit and function without individual part fittings; saves time and labor; not practical for some
many firearms require particular hand fitting to makes sure everything functions and looks good
bottleneck cartridges may need to establish headspace of firearm in final assembly
headspace - where cartridge sits relative to chamber; important in reliability and safety; if too tight, may not work when dirty; if loose, could misfire or cartridge could rupture when fired
parts usually checked for proper fit via physical feel or gauges
repeated firearms test fired with dummy ammunition to check for proper function
Final Steps
Proof Testing
done to stress test products
use special ammunition at pressures 30-40% higher than strongest commercial ammunition
in U.S. Sporting Arms and Ammunition Manufacturers' Institutes Inc. (SAAMI) does proof testing to adhere to voluntary guidelines
most other countries have government agency to proof test
every country has own rules of proof, but most follow European Commission Internationale Permanente standards
Test Firing
uses normal commercial ammunition instead of special high-pressure ammunition
show firearm functions correctly and meets accuracy standard
different performance standards between models and cartridges
number of test firings and accuracy standards vary by model and manufacturer
Serial Numbers
required on all manufactured firearms
can be added at any stage of production
must be on major component
1968 Gun Control Act - made serial numbers required fro low-cost commercial rifles and shotguns
Preparation for Delivery
residue from test firing removed
treat metal parts with rust inhibitor
firearm placed in padded container
commercial firearms packed with instruction manuals, warnings, etc.