Ricochet

A ricochet is a rebound, bounce or skip off a surface, particularly in the case of a projectile. The possibility of ricochet is one of the reasons for the common firearms safety rule "Never shoot at a flat, hard surface." Ricochets are sometimes called car[r]oms.

Variables
The likelihood of ricochet is dependent on many factors, including bullet shape, bullet material, spin, velocity (and distance), target material and the angle of incidence.

Bullet
Bullet construction has a major factor in determining both the likelihood of ricochet as well as where the bullet will travel afterward. Hard bullets have a greater tendency to penetrate than softer ones. Bullets that break up, such as varmint hunting bullets have a low risk of ricochet. The lower chance of ricochet is one of the reasons the newer .17 HMR round with its frangible bullet has gained popularity against the older non-fragmenting .22 WMR.

Velocity
Ricochets are often more common with low-power calibers such as .22 or .177 calibre, which can have trouble penetrating some materials, although a ricochet can occur with any caliber. This is because the low-power projectiles do not induce the same target responses as more energetic projectiles. Higher velocity projectiles have a tendency to either penetrate the target, and/or to break-up on contact with it.

Target material
Bullets are more likely to ricochet off flat, hard surfaces such as concrete or steel, but a ricochet can occur on almost any surface, including grassy soil, given a flat enough angle of impact. Materials that are soft, give easily, or can absorb the impact, such as sand, have a lower incidence of ricochet. Though it may not be intuitive, bullets easily ricochet off water; compare stone skipping.

Angle
The angle of departure, both vertically and horizontally, is difficult to calculate or predict due to the many variables involved, not the least of which is the deformation of the bullet caused by its impact with the surface it strikes. Ricochets will almost always continue on a somewhat diagonal trajectory to their original trajectory, unless the impact is against a flat surface perpendicular to the angle of incidence (or approach), in which case the angle of reflection depends on the other variables involved.

Dangers
Ricochets are a common hazard of shooting because the bullet that ricochets poses a serious danger of causing collateral damage to animals, objects, or even the person who fired the shot.

In rare cases, ricochets can return to the shooter. This can occur when the object struck possesses enough resistance to withstand the impact of the bullet, and its surface is perpendicular to the shooter. Alternatively, elastic targets such as rubber tires shot with lower power weapons can return the slug along the line of fire even when struck at an oblique angle. Some bullets are designed to deform at the nose, which is the main reason for the bullet ricocheting at such an extreme angle and returning in the shooter's direction.

Recommended reading

 * Federal Bureau of Investigation, "Bouncing Bullets," FBI Law Enforcement Bulletin, Vol. 38, Oct. 1969, pp. 1–9.
 * Garrison, D.H., "Crown & Bank: Road Structure as it Affects Bullet Path Angles in Vehicle Shootings," AFTE Journal, Vol 30, No. 1, Winter 1998, pp. 89–93.
 * Gold, R.E. and Schecter, B., "Ricochet Dynamics for the Nine-Millimetre Parabellum Bullet," Journal of Forensic Sciences, Vol. 37, No. 1, Jan. 1992, pp. 90–98.
 * Haag, L.C., "Bullet Ricochet: An Imperical [sic] Study and a Device for Measuring Ricochet Angle," AFTE Journal, Vol. 7, No. 3, Dec. 1975, pp. 44–51.
 * Hartline, P., Abraham, G. and Rowe, W.F., "A Study of Shotgun Ricochet from Steel Surfaces," Journal of Forensic Sciences, Vol. 27, No. 3, July 1982, pp. 506–512.
 * Jordan, G.E., Bratton, D.D., Donahue, H.C.H. and Rowe, W.F., "Bullet Ricochet from Gypsum Wallboard," Journal of Forensic Sciences, JFSCA, Vol. 33, No. 6, Nov. 1988, pp. 1477–1482.
 * McConnell, M.P., Triplett, G.M. and Rowe, W.F., "A Study of Shotgun Pellet Ricochet," Journal of Forensic Sciences, Vol. 26, No. 4, Oct. 1981, pp. 699–709.
 * Rathman, G.A., "Bullet Ricochet and Associated Phenomena," AFTE Journal, Vol. 19, No. 4, Oct. 1987, pp. 374–381.