Here is how to proceed to have arrows and darts for the crossbow in large quantities.

Skyrim Dawnguard - How to have infinite ammo for the crossbow

 

In DLC Dawnguard per Skyrim you can use a new powerful weapon: the crossbow. This is mainly used by the Dawnguard faction whose members, in their spare time, use it to go hunting for Vampires. The ammunition for this weapon (darts and arrows) is never enough and that is why we suggest a fairly simple method to accumulate a lot of it without spending anything.

Here is how to proceed to have arrows and darts for the crossbow in large quantities.

The first quest of the DLC will take you to Fort Dawnguard where you should find Agmaer in Canyon Dayspring. Take Agmaer with you to the fort. Here you do not complete the quest but explore the fort until you meet Agmaer again who, after finishing his conversation with Isran, should start training with his crossbow. He will continue to shoot arrows endlessly and you can continue to collect them by increasing your supply more and more. As you progress through the game and complete the subsequent quests you should still find Agmaer training in the area from time to time, so it will be possible to return to stock up on ammo for the crossbow.

The video below shows what has just been described. Homemade method, but effective!