Attack zombies with backpacks there the only zombies that drop stuff also they are mostly dropped in union island because it is the highest level place also have the M82 in your back pack while you are attacking zombies that usually increase the drop of .50 cal ammo.
i got the big heavy 50 cal M82 sniper rifle when i just farmed a whole lot of zombies at the end (before you hop onto the little boat) and ran to a safehouse and picked it up
First,for the people who don't know how to get M82 Sniper rifle:It can be obtained by completing 75% of the achievements.
Next,you can get more ammo by killing zombies with backpacks and searching all areas.Increase your luck skill(remember not searching skill) to get more chance to obtain from zombies(searching doesn't help in finding things from backpack zombies).Happy hunting.
I do not know if it works for you guys , but try to change the gun with his companion, then walk some maps and let him kill the zombies with the weapon , then take it back if it is with ammunition , then ran after changing a map ammunition will go away , then look in your bag and see if it appeared eight bullets came here . ( If have some mistakes in my English it is because I'm using the google translator :v , because is more fast for me )
