I guess I have to be the guy to point out that those bullets couldn't kill those things- it wouldn't even pierce their pelts.
A cylinder with a height of 1cm and a radius of 2.5mm (assuming by '5mm' he meant the diameter; if he meant circumference, it would be even smaller) has a volume of 1.96*10^-7 m^3.
The bullet material I will assume is lead, because that is pretty dense, at 11340 kg/m^3. 1.96*10^-7 m^3 of lead equals 0.00222264 kg (or around 2.2 grams).
While that is a pretty small bullet, the problem is the bullet is WAY too slow. 150 kph is only 41.6667 m/s.
Using the energy equation, 1/2mv^2, I conclude that this bullet would have only 1.93 Joules of energy upon initial release.
Compare this to the smallest 9x19 cartridge bullet, which is 7.45g (more than 3 times more mass) and can be fired at 360m/s (almost 10 times as fast) for an energy of 481J.
Hell, even if I give maximum leniency and say the radius is 5mm, the energy only goes up to 7.73J, which might be able to hurt people, but not these things.
Here is a exert from a Wikipedia article for the 2mm Kolibri pistol, which fires (I think) the smallest cartridge ever produced:
The cartridge weighs 5.3 grams (82 grains), measures 3 millimeters (0.12 in) at its widest point, and 11 mm (0.43 in) from the base of the primer to the tip of the bullet. The cartridge is headspaced on the mouth of the case. The bullet itself masses 0.2g (3 grains), and is estimated to have a normal muzzle velocity of 200 metres per second (660 ft/s), resulting in a muzzle energy of 4.0J (3 ft-lbs).[3] For perspective, an average person's punch measures 10-15 joules, meaning that the round has less energy than a punch.
The STEM in me had to write this essay. It was fun.