Forensic Ballistics Questions and Answers

Answer: B

The key gives due to lower loading density.

A hang fire is a delay between the firing pin striking the primer and the propellant igniting, so that the shot goes off late. The loading density is the amount of powder in relation to the volume of the cartridge case. When the density is low, the powder sits loose in a large air space and the flash from the primer may fail to ignite it at once, which gives the delay; reloaders report this for low loads in large cases, though I found no controlled study.

The other causes named for a hang fire are an aged or damaged primer or propellant, which the options do not offer. Option A, higher loading density, is the opposite of the keyed reading, and C is not a definite cause. Option D would deny any link to loading altogether. The key is the University's and rests on this reading.

Answer: A

The ballistic coefficient is directly proportional to the weight of the bullet.

\[ C = \frac{m}{i\,d^{2}} \]

Here m is the mass of the bullet, d its diameter and i its form factor. A heavier bullet of the same shape and calibre carries more momentum against air resistance, so it has a higher coefficient and keeps its velocity better.

The coefficient varies inversely with the form factor (a blunter bullet has a larger i) and inversely with the square of the diameter, so neither is directly proportional. 'Type of the bullet' is not a quantity in the formula.

Answer: D

The Greenhill formula assumes a bullet of specific gravity 10.9.

Sir Alfred Greenhill's rule estimates the twist needed to stabilise a bullet:

\[ \begin{aligned} &\text{twist (calibres per turn)} \\ &= \frac{150}{\text{length in calibres}} \end{aligned} \]

The constant 150 was worked out for a lead-cored bullet with a specific gravity of about 10.9, close to that of lead. For bullets of a different density the rule has to be corrected by the square root of the ratio of densities.

7.9 is about the specific gravity of steel and 8.9 of copper, and 9.9 is not the assumed value either.

Answer: A

The pressure developed in the .303 rifle is about 3400 kg per \(\text{cm}^{2}\).

The .303 is a high-velocity service rifle cartridge. Its chamber pressure is of the order of 20 tons per square inch, and forensic ballistics texts give it in metric units as about 3,400 kg per square centimetre. Pressures of this size are needed to drive its bullet at about 2,400 feet per second.

500 kg per \(\text{cm}^{2}\) is of the order of a shotgun's pressure, and 1000-1500 kg per \(\text{cm}^{2}\) belongs to low-velocity handgun cartridges; all are far too low for a rifle of this power.

Answer: A

Lead in discharge residue is best shown by X-ray radiography.

Lead is dense and strongly absorbs X-rays. A soft X-ray radiograph of clothing or skin around a bullet hole shows the scattered particles of lead from the primer and bullet as radio-opaque specks, and their spread helps estimate the range of firing without destroying the exhibit.

The dermal nitrate (paraffin) test detects nitrates, not lead, and gives false positives with many common substances. Infrared photography is used to show soot on dark fabric, and ultraviolet examination is not a test for lead.

Answer: C

Both (I) and (II) are correct.

The washing effect is the erosion of the bore, especially near the chamber, by the hot propellant gases rushing past the bullet at high pressure and temperature. They melt and carry away fine layers of steel, which is statement (I).

As this erosion builds up, the bore widens and the rifling wears away, so the bullet fits loosely, gas escapes past it and it no longer spins true. Accuracy (aim) and range both fall, which is statement (II). Because both statements describe the effect correctly, options accepting only one, or neither, are wrong.

Answer: B

The function of the primer is to ignite the main charge.

When the firing pin strikes the cap, the small charge of sensitive primer compound (lead styphnate, barium nitrate and antimony sulphide in many modern primers) explodes and sends a flash through the flash hole into the propellant, setting it burning. The primer is also the source of the lead, barium and antimony particles in gunshot residue.

It is the propellant that drives the bullet, not the primer. Lubrication is provided by bullet coatings or grease, and corrosion protection by the cartridge case and non-corrosive priming compounds; neither is the primer's function.

Answer: C

The blood in the barrel is due to the blowback effect.

In a hard-contact gunshot wound of the head, the gases of discharge enter the wound, expand beneath the skin and scalp, and rebound. Blood and tissue are forced back towards the gun and drawn into the muzzle and barrel; this is called blowback, and finding blood inside the barrel is evidence of a contact or near-contact shot.

Back spatter is the wider term for blood travelling back towards the shooter from an entrance wound, and forward spatter leaves through the exit wound. Impact spatter is the general class of stains made when force strikes a blood source.

Answer: C

The key accepts statements I and III, so the answer is I & III are correct.

A mixed or combination gun is a double-barrelled weapon whose two barrels are of different kinds. In the forensic ballistics classification this covers guns with barrels of two different bores (I) and, most typically, guns with one smooth-bore barrel for shot and one rifled barrel for a bullet (III), such as the drilling or cape gun.

A double-barrelled shotgun with different chokes in its two barrels (II) is an ordinary double shotgun, since both barrels are smooth-bored and of the same bore, so options naming II alone, or III alone, do not match the key.

Answer: C

Both statements are correct, so the answer is Statements I and II are correct.

Firearms examiners deal with a wide variety of weapons, from factory-made guns to country-made and improvised firearms (I).

Most of them nevertheless fall into four broad classes (II): handguns (pistols and revolvers), shotguns (smooth-bore guns firing shot), rifles (long guns with rifled barrels), and submachine guns (automatic weapons firing pistol cartridges). Since the two statements are compatible and both true, the options naming only one or neither are wrong.