Topic 3 - Forensic analysis of ammunition Flashcards
what is caliber and chambering and how does it describe ammunition size?
caliber refers to the diameter of the projectile and is the first number
chambering refers to the length of the cartridge and is the length of the cartridge and is the second number
additionally the chambering length may have a unique cartridge profile term associated with it
7.62 (cal) x 39mm (chambering) = Russian AK47 military
7.62 x 51mm = NATO military
7.62 x 54mm = German Mauser
what will happen if you use the wrong size ammunition ?
the firearms chamber is designed to have a set max pressure - a wrong sized round will rupture and cause shock front to impact the chamber which could lead to a rapid failure of the chamber/ breech area. the gun could explode.
what happens if you chamber a too big or too small round in a chamber?
- The round is too big for the chamber and
either will not fit or will jam in battery after
firing. - The round is too small, and so will chamber,
but is ‘loose in battery’.
what is a “headstamp”?
- The majority of ammunition has a maker’s
stamp on the base of the cartridge. - This is known as the “Headstamp”.
how would you investigate a cartridge case and identify where it came from?
try to identify manafacturer and genral design first
Examination of the headstamp may
reveal the caliber, and could also
provide manufacturer’s information.
the probable mechanisms should be relatively obvious from the size, shape and physical looks of the cartridge case.
how do you determine if a cartridge comes from a self loading mechanism or a single shot mechanism
- Cartridge shape is important when determining firearm type.
- Cartridges for self-loading firearms do not have ‘protruding’
base rims beyond the main body of the cartridge case. - Self-loading cases must have parallel sides to allow stacking in a
magazine. - Cartridges for single shot mechanisms, such as for a
revolver, do have a ‘protruding’ base rim. - This stops the cartridge falling through the cylinder of the revolver
- Cases are extracted via a ‘central web’, so need a rim.
- The two systems ARE NOT interchangeable.
what are the class chracteristics from a forensic perspective of a cartridge case?
general dimensions (caliber and chambering)
shape of firing pin mark
primer type
headstamp information
- These attributes may reduce an investigation down to the type and class of firearm that could have been used in a shooting incident, but not much more.
- To be more discriminating and to find the exact firearm in involved, we must look at the ‘individual characteristics’.
what are the individual characteristics of a typical cartridge case with marks?
ejection port dent
chamber marks
magazine lip marks
extractor marks
breech face marks
ejector mark
firing pin impression
firing pin scrape mark
slide scuff mark
slide drag mark
how would you analyse individual characteristics in a lab?
use a comparison microscope or a modern 3D surface scanning technology such as the IBIS BRASSTRAX with MATCHPOINT.
what is a bullet?
- any projectile below about 20 mm in diameter that
is fired from a barrelled weapon. Above this, it is a ‘shell’. - Normally has a lead core.
- The core is often surrounded by a gilding metal jacket.
- Gilding metal is a form of brass (an alloy of copper and zinc) with a much higher copper content than zinc content.
- Remember: The bullet is the bit that moves…the rest is the cartridge and together with propellant/primer, a round
what is the caliber?
- The caliber is the effective diameter of the bullet…NOT that of the cartridge.
what is the typical velocity of handgun bullets?
Handgun bullets: 200 – 400 ms-^1
what is the typical velocity of rifle bullets?
Rifle bullets: 300 – 1500 ms-^1
what materials is a bullet made of?
- Usually a cast lead antimony alloy core.
- Typically: 97% Lead, 3% Antimony.
- The antimony (Sb) reduces the
malleability of the alloy and makes the lead core less easily deformed. - Most modern bullets are fully or semi-jacketed to protect the softer lead core.
- Un-jacketed bullets are still in use though for low velocity applications.
Why would you put a jacket around the lead core of a bullet?
- Jacketing stops ‘rifling drag’ in the barrel at
higher velocities. - Lead is so soft that the bullet skids over the
rifling rather then engaging with it. - Lead also deforms readily under acceleration.
- A gilding metal jacket improves rifling pick up,
and reduced acceleration deformation.
