First go to you project settings, then to the physics category. The scroll down to the physics surface list, here you can add new surface types for materials like wood metal stone etc. Start by changing the first None surface type to the name of your surface for this guide we will use wood. So it should look like the image below.
Next go to materials folder then to the Physics Material folder right click and go to the physics category and select the Physics Material, then select the physics material option in the window that opens and press the select button.
Then you can rename this materiel to what ever you want but I will call mine PM_Wood. Now open the materiel.
Then in in the physical Properties category there is a drop down option you can select you new wood surface type. (If its not there you may need to restart the engine for it to show up).
Next you can go to any wood materials open them and in the details panel there is an option called Phys Material, there you can set your PM_Wood. This apply the physics material to any place this is used.
Next go to either you melee or range weapon holdable bp, for this guide i will use the BP_M4A4 once open in that bp then search for decal and it will bring up the Surface Decals variable that will look like the image below.
Next you can follow the video below. I dont have another decal but this will show you how to set yours.
You can see in the video I delete the default surface decals in the video this is because you can add new one while the default option is there, Then I add my new wood surface decal then i readd the default one and set its decal material again. If you dont have the default option there already then you dont need to worry about this.
For emitters you just search for surface emitter and repeat the last step as shown in the video, by pressing the + setting the drop down option to wood then set the particle effect to your chosen effect.
Now anything using a material that has phys material set to your PM_Wood the weapon will use your decal and emitter effect when it shoots / melee attacks it.