The same airsoft bb gun can fire both plastic spherical projectiles and metal spherical projectiles (steel balls), as well as spherical projectiles of other materials. A few airsoft bb guns can also fire airsoft bullets[1] (lead bullets). These different materials and forms of projectiles have different muzzle specific kinetic energy and different wounding capabilities. The wounding ability of plastic projectiles fired by an inflatable BB projectile gun is less than that of metal and glass projectiles of the same diameter[2]; compared to plastic projectiles, metal projectiles have lower muzzle velocity and muzzle specific kinetic energy. Rise [3]. In the "Forensic Science Muzzle Specific Kinetic Energy Velocity Test Procedure" (GB/T 953-2011) [4], it is stipulated that "If a gun can fire projectiles of multiple materials or forms (such as plastic projectiles or metal projectiles, etc.), The maximum specific kinetic energy shall prevail" to determine whether it is a legal gun. If the muzzle specific kinetic energy value of projectiles of multiple materials or forms exceeds 1.8 J/cm2, whether the above regulations are reasonable has caused confusion to the appraisers. Therefore, from the perspective of further standardizing the testing work, it is necessary to deeply understand the above-mentioned law of the difference in the specific kinetic energy of the muzzle.

1 experiment

Using the method of supplementary inflation, always keep the gas in the inflatable magazine at the maximum pressure, so that the airsoft bb gun fires plastic, glass and steel spherical projectiles (steel balls) of the same diameter with the same initial launch energy, and measures the speed of the three. Calculate the kinetic energy and specific kinetic energy of the three, and analyze the relationship between the speed, kinetic energy and specific kinetic energy of the three.

1.1 Experimental equipment

The 6 mm shape imitates MP5A4 inflatable gas submachine gun (Figure 1); "Mengjiang" brand inflatable tank [2], the main component of the gas in the tank is fluoropropane; 6 mm (the measured average value is 5.90 mm) plastic spherical airsoft bb, Glass spherical bullets and steel balls, the masses are 0.21, 0.26, 0.87g respectively, and the surface of the three types of projectiles is smooth; Model 36 speedometer made in the United States, bullet arrester; balance; tape measure; vernier caliper, etc.

1.2 Experimental process

The experiment site is the ground floor, and the room temperature is 25 ℃. The gas-filled tank is stored in the experimental place for a long time, and the gas temperature in it is the same as room temperature.

Inflate the inflatable magazine of the experimental gun with the "Mengjiang" brand inflatable tank (full tank) until no air intake sound is heard, and then stop charging for a few seconds; put the speedometer in the ready state; launch in sequence 1, 4, 7 Plastic spherical bullets, launch glass spherical bullets in sequence 2, 5, 8, and steel balls in sequence 3, 6, and 9, that is, the first plastic bullet is launched, the second glass bullet is launched, the third steel ball is launched, and the fourth bullet is launched Launching plastic bullets...; the speedometer automatically records the data; after each bullet is launched, the inflatable magazine is filled with an inflatable canister until the air intake sound is not heard, and the charging stops for a few seconds.

2 Results and discussion

Set the shooting distance (the distance from the muzzle to the first target plane of the speedometer) respectively 30, 100, 200, 400 cm. Collect 3 rounds of data for each of the 3 material projectiles at each shooting distance, a total of 36 data. Average the data of projectiles of the same material at the same shooting distance to obtain the data in Table 1.

Take the data in Table 1 with a shooting distance of 30 cm and plot it into Figure 2. It is not difficult to find from Figure 2 that as the mass of the projectile increases, the speed of the projectile decreases and the kinetic energy of the projectile increases, that is: the lighter projectile has a higher speed and lower kinetic energy, while a heavier projectile has a lower speed and higher kinetic energy. . Since the diameters of the three projectiles are the same, the greater the mass of the projectile, the greater the specific kinetic energy of the projectile. The relationships among the mass, speed, kinetic energy and specific kinetic energy of the projectiles at 100, 200, and 400 cm shooting distances in Table 1 conform to the above-mentioned law.

Figure 3 is a schematic diagram of the speed of three kinds of projectile changes with shooting distance. It can be seen from the figure that as the shooting distance increases, the smaller the mass of the projectile, the faster the speed decay. Due to the aftereffect of the muzzle gas, the speed of the plastic and glass bullets reaches the maximum when the distance is about 100 cm from the muzzle, and then gradually decreases; the speed of the steel ball at 30 cm from the muzzle is lower than the speed at the other 3 shooting distances When the shooting distance is between 100 and 400 cm, the speed rises slightly and almost remains flat. This is because the steel ball moves slowly, and the aftereffect of the muzzle gas is relatively balanced with the air resistance of the steel ball.

Figure 4 is a schematic diagram of the specific kinetic energy of the three projectiles changing with shooting distance. The specific kinetic energy of plastic bullets and glass bullets reaches the maximum at a distance of about 100 cm from the muzzle, and then gradually decreases; the specific kinetic energy of the steel ball gradually increases at a shooting distance of 30 to 400 cm, which is the result of the aftereffect of the muzzle gas. At the same shooting distance, the specific kinetic energy of the glass bullet is greater than the specific kinetic energy of the plastic bullet, but the difference is not much; the specific kinetic energy of the steel ball is also greater than the specific kinetic energy of the plastic bullet and is much larger. The ratio of the two is 1.59 (shooting distance) 30 cm), 1.63 (100 cm shooting distance), 1.71 (200 cm shooting distance) and 2.02 (400 cm shooting distance).

Where: ug—the velocity of the projectile muzzle; p0—the initial pressure of the gas injected into the barrel of the air gun; v0—the initial chamber volume of the barrel; γ—the adiabatic index of the gas used for launch; φ 1—the coefficient of minor work; M — The mass of gas injected into the barrel of each bullet; m—the mass of the projectile; s—the cross-sectional area of the barrel; L—the length of the barrel. γ, s, and L are constants. In this experiment, p0, v0, and M can be regarded as constants. The secondary work coefficient φ 1 here mainly reflects the coefficient of work done by the friction force of the projectile. Since the surface of the three projectiles is smooth and the matching clearance between the projectile and the inner wall of the barrel is the same, the friction force of the three projectiles in the barrel is the same, so φ 1 is also constant. From equation (1), when the mass m of the projectile increases, the left denominator of the root sign increases, so the muzzle velocity ug of the projectile decreases.

Transform formula (1):

Equation (2) is the internal ballistic calculation formula of muzzle kinetic energy. When m increases, φ 1+M/(3m) decreases, so the muzzle kinetic energy increases. From the above calculation, it can be seen that when the mass of the projectile increases, the muzzle velocity of the projectile decreases and the kinetic energy of the projectile increases; conversely, when the mass of the projectile decreases, the muzzle velocity of the projectile increases and the kinetic energy of the projectile decreases. This conclusion is consistent with the above experimental results.

3 Conclusion

When using an inflatable air gun with the same energy each time to fire spherical projectiles of the same diameter with different materials, the greater the mass of the projectile, the greater the specific kinetic energy at the same shooting distance. In other words, the greater the density of the spherical projectile material with the same diameter, the greater the specific kinetic energy at the same shooting distance. When testing the specific kinetic energy of the muzzle of this type of air gun, if the specific kinetic energy value of the plastic projectile has been determined to be greater than or equal to 1.8 J/cm2, there is no need to measure the specific kinetic energy of other materials with high density glass, ceramic or steel ball equivalent diameter spherical projectile , Because the specific kinetic energy of a spherical projectile of the same diameter with a higher density is greater. When the test value of the muzzle specific kinetic energy of a projectile of multiple materials or forms exceeds 1.8 J/cm2, a certain projectile should be determined as a test projectile, and the muzzle specific kinetic energy value of the test projectile should be used to determine whether it is a gun.