#BC

When I started the Modern Intermediate Calibers series, I did not expect it to grow as large as it has. The initial plan was for 7 major calibers, which grew into well over 20, and the spinoff Modern Personal Defense Weapon Calibers, which itself will have at least 20 entries.

In the late 1950s, after the first public demonstrations of the AR-15 and its new small caliber, high velocity cartridge, the Soviet Union took notice of the radical developments in military .22 caliber rounds in the United States. By 1959, four years before the adoption of the AR-15 as the M16 by the US Army, Soviet ballisticians were already testing Soviet-made replica 55gr spitzer FMJ bullets fired at over 3,000 ft/s from modified necked down 7.62x39mm cases. This program for a new small caliber high velocity lasted into the late 1960s, but it wasn’t until the mid-1970s that the 5.45x39mm caliber was eventually issued alongside the AK-74 rifle, a modified but significantly more effective variant of the previous 7.62x39mm AKM assault rifle.

In the second installment of Ballistics 101, we took a look at the concept of a “ballistic coefficient”, or a drag model based on an empirically tested projectile, against which other projectiles can be measured and compared. Mentioned in that post – but not explained – was the concept of form factor, which is the subject of today’s installment.

When a bullet flies through the air, forces called “drag” are exerted upon it. These forces slow down the bullet as it flies, but they don’t act the same way on every bullet. While the subject of aerodynamics is extremely complex, one way to account for differences in bullet drag that is commonly used in the world of small arms is the “ballistic coefficient”.

Recently previewed at SOFIC 2014 in Tampa, Rianov has been putting the finishing touches on their latest ballistic computer, the Eagle II.  Designed with high-speed, low-drag in mind, the Eagle II can store dope information for up to 16 different weapons.