the title speaks for itself… but i wonder if there is a clear velocity that UE is basing its add velocity module on.
this might be helpful when i want to use it for different bullet types for realistic shooter games (although you would set the velocity up in bullet BPs not in niagara)
anyways, just curious. would be nice if someone could clear this up 
Niagara uses the same “1 UE unit = 1 centimeter” convention, so if you set a linear (1,0,0) velocity and 1 velocity speed scale in Add Velocity module, it moves the particle forward 1 centimeter per second (if there is no other velocity module in the same emitter). If you want to set “meter per second”, just multiply the speed scale by 100.
Like you have already said, you would do the bullet in BP instead of Niagara, because Niagara event handler can be quite taxing. If you attach the trailing Niagara effect on the bullet and need to refer to the bullet’s speed in Niagara, you can use the “Engine.Owner.Velocity” which is just the resultant (world space) velocity of the actor.
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thank you! cleared all my question about this 