Here's what I did on one of my shields. A benefit of this technique is you apply it over the clearcoat and don't remove the paint with acetone to get the silver.
Getting the effect was really simple. To get the bullet hits I held a can of flat black spray paint 5-6 inches from the shield and quickly fired a shot. Make sure you use a paint can that has a vertical thin spray tip, not a round one. I then dry-brushed the silver scrap in the middle of the black. The rest is various weathering around the shield. I didn't want to actually scrape or scuff the shield (damaging the metal itself) in case I change my mind and want to undo the weathering.
I also weathered the back. Mostly dry-brushing around the metal to make it look worn.
Hope this helps.