If you're running the survival loop, you need an upgraded shotgun or thundergun as your oh shit weapon. You don't upgrade your mp40 for obvious reasons
Best gun (by far) is the HK-21 upgraded. It has OVER 125 bullets per clip and it's damage is amazing. Use that one for your upgrade, if you want to get far in KdT.
lmgs are good in the beginning to get pts but after tht they suck so dont PaP them, ar's arent much good, smg's are alright for later if you need pts, you want a ray gun, thundergun or a crossbow in the later rds