Ballistics: Building and using the ballistic pendulum. We end the journey into ballistics, describing the construction and use of a precious instrument to measure the speed of the projectile: the ballistic pendulum.
In a previous article we described the ballistic pendulum, the instrument for measuring the speed of a bullet and which replaces the chronograph.
DIY enthusiasts can easily make a ballistic pendulum at home, without having to rely on expensive commercially available chronographs. In addition to the material realization of the pendulum, we will also describe its use in order to have a complete picture of this extraordinary ballistic instrument.
First of all, the ballistic pendulum must necessarily meet the following requirements: it must have a length of 1.5 meters in order to make the oscillation adequate and slow. Secondly, the pendulum mass must have a weight so that it does not oscillate more than 20 °, in relation to the bullet fired, and must be suspended in such a way as to be able to oscillate in a single plane without skidding, rotational movements and parasitic oscillations. To be precise, the ballistic pendulum must have a weight, expressed in grams, equal to the product of the speed of the bullet in m / sec by its weight in grams. Basically it will weigh about 1 kg for the cal. 22, kg 1,5 for the cal. 7,65 and the cal. 9, and so on.
Thirdly, the block must be constructed in such a way that the bullet can penetrate and remain embedded; for some types of projectiles a block of wood will be sufficient, while for others it will be necessary to use, in addition to wood, also layers of lead or other metal, with which complete perforation is prevented. To make the block, any metal box in the shape of a parallelepiped or cylinder, in which the layers of wood, sand and lead are placed, may be sufficient. Finally, the block is held suspended through four thin and inelastic threads, joined two by two with rings to any stable support.
Under the box, more precisely on the extension of the perpendicular axis of the pendulum, a sliding index must be fixed passing through an arc equipped with a metric scale. To measure the maximum displacement of the index finger due to the shot, it will be necessary to study any system, from the fine brush to the tip of the index, to the wire dragged by it, to the powder brushed by a "whisker", which does not offer resistance to motion .
The do-it-yourself realization of the pendulum requires precision, study and dexterity and will also save a lot of money to those who want to test their own cartridges.
Let us now explain how the ballistic pendulum is used.
The first step is to weigh the block accurately. Then you have to shoot against the block, at a distance that avoids the influence of the firing gas (or through a perforated diaphragm) trying to hit the center of the block, in order to avoid irregular oscillations. It is useful to remember that with each shot following the first, the weight of the block must obviously be increased by the weight of the previous bullets fixed in it, also taking into account any loss of material caused by the impact.
After the shot, the maximum displacement of the pendulum must be read, since the speed of the projectile is derived from this reading: the latter must be done with a certain precision because, for example, with a pendulum of 130 cm in length, a reading error of 4 mm (10 ′ degree), leads to an error in the calculation of 5-6 m / sec.
An example will make it clearer. To measure the velocity of a 9 cal short bullet you must use: a pendulum of gr. 1500 of weight if the bullet is from gr. 6; the theoretical length (l) of the pendulum must be 1,20 m, while the total length (L) must be 130 cm; path arch of 40 cm.
Applying this formula we find the alpha angle:
and therefore:
If you have fired at the pendulum from a distance of 1,5 meters, and taking into account the internal resistances of the system, you get a fairly exact value of the initial speed by increasing the one found above by 1%.
Ultimately, the ballistic pendulum is an excellent instrument that can easily replace the chronograph in many of its functions. The hunter keen on ballistics and gifted with good dexterity can easily build and have fun using the ballistic pendulum to carry out tests at different distances and with different projectiles.
We remind you that this article is limited to training the reader of the basic notions on the realization of the ballistic pendulum and therefore we invite those who do not have the appropriate scientific knowledge, specific technical equipment and places - shooting ranges suitable for carrying out ballistic tests with real weapons. strongly refrain from making the ballistic pendulum.