How to use an entity's impact to create a decal?

I have made a projectile, and i would like it to create a decal when it hits the wall, so i tried putting it in the AddCallback function,

local trace = util.TraceLine({ start = entity:GetPos() , endpos = dat.HitPos , filter = {entity} ,  })
			local Pos1 = trace.HitPos + trace.HitNormal
			local Pos2 = trace.HitPos - trace.HitNormal

but it just bounces

by the way the projectile is a rectangular prop with a tf2 needle effect parented to it i think thats the issue idk