The 50 Cal Machine Gun can be found on the back of a pickup truck on the highway between the Shooting Range and the Scouts Camp in the Toxic Valley. So far this is the only location I have found in my own adventures.
The easiest way to kill a super mutant is to target one of it’s legs. It takes 1 shot from a 50 damage rifle to cripple the limb which always drops the Super Mutant’s health down to less than half and the second shot on the same limb will finish off the job.