on the subject of shell holes, making a start using a Dremel mini drill (other makes are available) to make the holes, coating the inside of the holes with Mr surfacer, when this is dry and hardened off I can re-create the texture of broken armour with the tip of a file.
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using the real thing for reference, you can just make out striation marks in the armour from the driving bands of the shells, something to think about reproducing in miniature.
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I used to make holes using a hot soldering iron.
The resulting ridge of melted plastic around the hole was a very good representation of the ring you get around armour penetrations, such as in the photo.