SMGs are like stronger pistols. They are also light, their damage is low, but the shooting speed is far better, thus with certain perks and improved penetration they can show a good DPS. Entry-level submachine guns like Genesis or Siberia may seem unreasonably weak, it's because with their short range you have to be close to the target to do the full damage and because they need a lot of perks to become powerful. Like many pistols, SMGs also have an experience bonus (many of them, but not all, and not on high levels). High-level SMGs (for example Zion or Walloon) are a completely different story, they more like tiny miniguns, with all the DPS, but without heavyweight. Take the special perk Recon if it's offered to you, it is going to make SMGs even more powerful.
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