Friday, January 14, 2011

Testing Firearms: Measuring Bullet Velocity - III

In our last post, we studied a few methods that measure the time taken by the bullet to travel a known distance and thereby calculate its velocity. These methods fall under the class of chronograph methods. All these methods were generally inferior to the ballistic pendulum method for a variety of reasons, chiefly the inability to measure small amounts of time accurately and also the inability to ensure that the targets were moving at uniform rates.

While the ballistic pendulum method was superior for a long time, one of the issues it had was the weight of the apparatus. For testing the velocity of ordinary rifles or shotguns, the ballistic pendulum alone needs to weigh around 25 kg. (55 lbs.), without considering the weight of the supporting frame. When people try to scale this method for larger cannon balls, the weight of the pendulum apparatus increases exponentially. For example, in 1781, one Mr. Hutton tried to measure the velocity of cannon balls weighing just 3 pounds and his pendulum weighed about 315 kg. (approx. 700 lbs.). During the period of 1842 to 1847, one Major Alfred Mordecai from the United States Army tried to determine the muzzle velocity of larger guns and built a ballistic pendulum weighing over 4215 kg. (approx. 9300 pounds) and was mounted between two large brick towers. This could only measure velocities for 32 pounders at most. It was estimated that to build a ballistic pendulum to measure velocities for even larger weapons, one would need to build a massive pendulum suspended by the two Brooklyn bridge sized towers!

Meanwhile, the discovery of electricity made chronograph methods much more accurate. It was now possible to measure the beginning and end of a period of time using some sort of electrically operated mechanism. It also became possible to measure very small intervals of time accurately, making chronograph methods much more accurate than was achievable previously.

One of the early chronoscopes was invented by Charles Wheatstone, a noted scientist of the Victorian era. Among his other inventions were a stereoscope, an encryption system, several developments in telegraphy and the wheatstone bridge. His chronoscope consisted of a wooden ring fixed to the muzzle of a gun, with a thin wire running through the middle of it. The target was placed at a known distance and consisted of two plates which were arranged so that the least impact would result in a permanent contact between the two plates. The wires were hooked to an electromagnet mechanism and a small battery. Initially, a continuous circuit would be maintained. Then when the firearm would be discharged, the bullet would leave the muzzle and pass through the wooden ring and cut the thin wire running through the center. This would deactivate the electromagnet, which would then start a special clock driven by a falling weight. When the bullet would hit the target, the second circuit would be completed, which would re-energize the electromagnet and stop the clock. This mechanism was capable of measuring time with a resolution accurate to approx. 1/7300 of a second, which allowed it to calculate bullet velocities very accurately.

A more modernized version used a tuning fork to measure small increments of time. The tuning fork would have a thin stylus attached to one of the arms and a roll of paper would be gradually moved over the stylus. The tuning fork would be activated and deactivated by electricity and the tester would count the number of vibrations of the tuning fork, inscribed on the paper roll, to determine the elapsed time.

Using electricity to measure time became much more popular because the same setup could be used on just about any caliber firearm.

These days, modern chronographs use optical detection or to determine the passage of a bullet through a known distance. For instance, photo-transistors (such as those that work with infrared frequencies) could be used to start and stop a highly accurate stopwatch. The passing bullet shadow causes the circuit to be activated and deactivated at the two ends of a known distance and very high time resolutions can be obtained. Such devices are pretty cheap as well and available for around $100-$200 or so.

In the above images, we have a modern chronograph that can measure bullet velocities between 30 - 7000 feet/sec with 99.5% accuracy. The V arms indicate the area through which the bullet should be shot for the sensors to detect it. The two white strips on the top are light diffusers, so that this device can be used even in bright sunlight. They also help to provide a uniform background so the photo-transistor sensors can easily detect the contrast a passing bullet. In less than bright daylight conditions, the two diffusers can be optionally removed. The digital display automatically calculates the bullet velocity, so all the user needs to do is position this device on a flat surface, such as a table, turn it on and then shoot through the two V areas. High velocity rifles should be shot from at least 3 meters (10 feet) away and lower velocity weapons may be shot from around half that distance to get accurate readings.

Modern chronographs such as the one above make it a breeze to calculate bullet velocities. Due to their lightness and low cost, these are overwhelmingly the method of choice to measure bullet velocities these days.

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