M870 shotgun with 5x sentry the 6 gear item being Vigorous chest or savage gloves.
That goes for both PVE and PVP.
For PVE that is pretty much the best unless you want to use other SGs like SASG.
For PVP you can make SMG builds too and they can be made really many ways but you want MP7 for that.
MP7 is ok, but its basicly a poor man's LOVA / lightweight M4. If you like being close and on point, you want responsive on your weapons. Its a dmg bonus you get when at 10m or less when shooting.