When it comes to the sword Thin Precipice it's ammo is currently glitched.
Currently the sword doesn't work with ammo reserves or enhanced chain reaction and also the sword always gives you 1 less ammo than what it says so if you should have 58 ammo it will only give you 57 and so on.
Hopefully Bungie will look at this and fix it.
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The ammo counter being off by one doesn't seem to be just swords, I've noticed that on several weapons lately. Mostly crafted guns. I'm guessing it's a rounding error causing a visual hiccup for mag/reserve size.