My idea is simple - get rid of the tracking rounds, make it hitscan and ditch the 3-round burst in favour of full auto. For me this gun is so dire it needs a serious buff.
[spoiler]It could even herald a new special weapon type, the SMG, that bosses the area between shotgun and fusion range.[/spoiler]
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That sounds pretty boring. Exotics should feel exotic. Turning the Promise into just a full-auto sidearm isn't particularly fun or interesting.