This has been a problem in beta builds, and it still is, apparently. Simply put, SMGs and Sidearms need more ammo.
Submachine Guns have reduced ammo capacities compared to Auto Rifles.
Given their rate of fire, Submachine Guns should carry more, or at least as much ammo as Auto Rifles. The ammo inventories for SMGs in their current state are just plain asinine.
Also, Sidearms as much ammo as Hand Cannons, but they're more like Scout Rifles in terms of damage. They should carry Scout Rifle levels of ammo, given their reduced damage.
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Since they are both close range weapon, they don't really need that much ammo. You should be able to kill 1-2 enemies for every magazine if you don't miss half the shots