Step 1: Naviagte to root/usermaps/scripts/zm/mapname.gsc
Step 2: Below this line:
Add this code
startingWeapon = "ray_gun";
weapon = getWeapon(startingWeapon);
level.start_weapon = (weapon);
Make sure to change ray_gun to the weapon of your choice. List of current weapons included in the stock map can be found here: https://confluence.ugx-mods.com/display/UGXMODS/BO3+%7C+Weapon+List
Step 4. Save and close, then check the Link box when building your map or else changes WILL NOT TAKE EFFECT!
Done! You have successfully changed your starting weapon!