My decal looks like a black square when it should be a circle. the decal gets spawned when my hitscan weapons hit something or my projectiles hit something.
screenshots might be useful. Try to drag a decal actor into ur scene, apply the material and see what it looks like. If it looks weird, you probably need to rotate it and that might even solve your problem, spawn the decal aligning to the surface. If it does not work, you might have a problem in ur material setup.
actually I found the answer! 
What was the answer?
It’s likely that, the rotation of the decal wasn’t correct.
Rotate the decal by 90 degrees on one of it’s angles.
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