Annihilation does not always create photons. However, if photons were created than momentum would have to be conserved. If the bullet and antimatter had equal but opposite momentums, then the simplest case of photon production would be to create two photons going in opposite directions at the same momentum. This is because momentum in the original frame was zero, and a single photon cannot have zero momentum from that frame. Light always moves at the speed of light from all frames.
The mass would be converted to energy through mass-energy equivalence, and additional kinetic energy from before the collision would also have to be conserved. Usually the velocities are so small relative to the speed of light that we ignore the kinetic energy as most of the photon energy is from mass annihilation. If the kinetic energy was significant it would be conserved in the created photons by increasing their frequency and thus momentum. The energy of a photon is known by the equation $E=hf$ with h being Planck's constant and f being the frequency.