Once you have the Dual Vipers upgraded to Dual Raptors, equip Jackpot mod (two if you have them) and Shock mod. Go to the Endzone Challenge in the Battledome. Use the Raptors repeatedly. This is a good place to train any weapon. If you are doing the challenge with Dual Raptors, you should get a large amount of bolts quickly (about 6 million per play).
Use the Magma or Vulcan Cannon and equip the Jackpot mods (as many as possible). Do the Endzone Challenge. Stand a good but close distance from the starting line and repeatedly fire. It is possible to get close to 3 million bolts.