Too much sidearm spam, sidearms are being used as primaries. Blah blah blah. Although i'd imagine this is what bungie intended.
So bring them in line with other secondaries, they are by no means overpowered because of weapon design, they're overpowered because you can stockpile ammo. Take that away, please. Primary meta should be a PRIMARY meta
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The sidearm is basically a primary in the special slot so I think the ammo is fine as is.