I often see people asking for SMG builds, or for help for them, with the default assumption of high dex and high per with low str. I think I have a superior SMG build, though I've never played it myself.
https://underrail.info/build/?HgcQBgMGAwVQHgDCoB4AwqAAAQEAAQEBAQEAAAAAAQAAwrM5Jk87FsKjN8KFAkkVKA1OEsKD4p-iBeKijQPip6oD4qimBN-_(feats arent in order; thats up to you)
Max dex, max melee, versatility. Other abilities to meet feat prereqs, namely full auto, sprint/spec ops, suppressive fire, and versatility. This gives you minimal AP costs for SMGs, gets you all the feats you'd ever want for them, dirty kick (a melee attack that doesnt require a weapon slot and will stun enemies), and even expose weakness for those really tough enemies that SMGs may have trouble with. Your damage per bullet may be a bit lower, but you're spitting out even more bullets, which is the point of SMGs, as I understand it. Your initiative will be high, you'll get massive skill boosts to dex skills (throwing, lockpicking, pickpocket, traps). SMGs need a lot of bullets, should you ever run out you can use melee weapons.