Look at weapons showcases for MW2019, | Official Unity Discord | Page 1

This is not the old school method of using lerped trail renderers to visualize where a raycast bullet hit, as that type can't do gravity without a bunch of extra work

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So they're particle renderers of some variety
If the bullets themselves are projectiles with gravity, none of that simulation needs to be done by the particles and they only need to be attached to the bullets

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Might be circular particles with trails, or circular particles of the "stretched billboard" type that are round from behind but elongated from the side when in motion
Meshes work too but mind the polycounts

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If you need a lot of particles and to attach them to projectiles that already are simulated on the C# side then VFX Graph is the most effective option

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If the particle positions only need to be approximate and handled by the particle simulation, then Particle System is an option as well, being easier to use than VFX Graph in my opinion

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For rapidly firing weapons games often don't bother simulating every tracer or even matching them up with the real bullets

#Look at weapons showcases for MW2019,