I found this out while tryin' to save up some cash, just blowin' off some time. I'm not sure if anyone else has noticed this but it's great.
First, you need to have challenge mode started and be just before the Vindicator tournament I think. The challenge you want is called "Endzone".
Bring with you the Dual Vipers, equiped with as many Exp. Booster mods as you can afford at the time, No worrries, it'll all come back. Any slots you have left you may want to add some Ammo mods.
Equip the Shock Omega mod for easy killz for the smaller enemies. Try not to get hit at all or your bolt multiply thing goes back down to 2 an' starts over. Enemies often drop ammo into the brown stuff or happen to leave it where you can get it easily. Just use the strafe thingy an' strafe the sides an' you will get enough ammo for the challenge.
You literally get 5,000,000 bolts per challenge this way, no lies.
-SGX
Baby I like it.