Couple, actually. One is called the Turret mod and adds a handful of cool turrets, but mainly for PvE. Another is the TF2 Turret mod, and I don't know if it'd even work for PvP. The modded Father Toast has a turret mod, but again, simple it kinda isn't, and seems to be only for PvE.Isnt there a mod that adds turrets to the game for defense?
Really, though, OP, you could just use MFFS to protect your base at chokepoints and against ICBMs if you are worried about that, with a few fortron generators and a Railcraft steel tank to store the liquid fortron. Yes, it's a liquid. Yes, that means one can defend many places with a single source of fortrons.