[inspired from the bolt pistol from wh40k darktide]
https://www.reddit.com/r/destiny2/comments/1gtqgec/what_if_we_got_a_bolt_pistol_like_exotic_in_in_d2/
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Rocket assisted side arms are probably the closest we'll get to bolter rounds.
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Erianas Vow, Forerunner, Rocket Sidearms as mentioned before. All of these hit like a truck and fire pretty slow. (Forerunner being the exception). We already have it.