It's been around for at least 20 years. May as well record it.

Not in any of the usual refs. A twitter search shows dozens of usages per day. I see it used in reference to both flesh wounds and bullet holes in walls, cars, etc.

https://youtu.be/NNVtF8kvaEY?t=272
今 日本で銃痕みれるとこなんてなくて

https://yuran.work/tokyorevengers-228-netabare/
ワカ「！」「銃痕(じゅうこん)!?」

https://kensanbiog.com/9311
壁を撃つと銃痕（じゅうこん）が表現されます。

https://en.wiktionary.org/wiki/銃痕
銃痕(じゅうこん) (rare) gunshot wound, gunshot hole


Google N-gram Corpus Counts
╭─ーーーーー─┬───────╮
│ 銃痕　　　 │ 2,888 │
│ じゅうこん │   211 │
╰─ーーーーー─┴───────╯

Web search results mostly turn up usages in anime / manga / etc. This might be an incorrect or colloquial version of 弾痕