If you haven’t already you should follow the guide for the “Add emitter effect to range weapon” guide as this guide relays on code from that guide.
Go to the BP_MasterWeapon and create a variable called Surface Decals. Set its type to EPhysicsSurface and follow the video below.
Next add the highlighted nodes shown in the image below
If you dont have your own decal materials for the bullet holes. You can go to the learn tab of the Epic games launcher. Once you have downloaded it you can migrate the M_Impact_Decal material to the survival game kit. and use that in the Surface Decals variable.