When the Carcaj is placed into a crafting square, it turns back into 9 arrows. This makes it an efficient storage method. It is advised to bring 20 to 30 Carcaj items with you when you travel on long journeys, that is, if you expect to run into many enemies.
It is incredibly useful once (normally on board pirate flying ships, over the ocean) the Long Bow is found, a better dungenous item that shoots arrows much further away. This bow can shoot arrows directly from carcajs, meaning that one can have 1 stack of carcajes in ther invintory, not 9 of arrows, making it very efficient. The longbow does not lose durability, and although it cannot fire all the other cool types of arrows (like explosive, rope arrows, fire, ice, cool stuff), it can somehow be enchanted with dyes, which give it power based on the dye. for example, some dye can give it fire, some can give it explosive, etc. And it will do this effect for all arrows fired from it. (Further Testing Needed)
Long Bows can also be enchanted.
One odd thing about Carcajs is when fireded from a Long Bow, when 1 is in your invintory, it will lose durability. (as 1 arrow is gone). at 9 arrows it is gone, as all "durability" is lost. However if you fire from a STACK of carcajes, it shows the stack, and the loss of druability. If you were to take one of those carcajes out, or any ammount, you would see every carcaj in that stack has lost durability. as soon as you have used up the (top) carcaj, all the others spring back to full durability. so: They all TECHNICALLY have full durability except the one, but if you were to have 2, and take 1 out partway through, you would have 2 with half durability, wasting about 4 arrows (not much, but hey) Vise Versa, you can only put carcaje stacks of the same durability togeather (when not full stacks). You can also put these stacks in a Thaumic Restorer, and it will charge you (if you will) to restore every one. So if you have a whole stackwith 1 arrow left, just fire the damn arrow, pick it up, and throw it into your crucible, instead of wasting tons of Vis restoring EVERY arrow. Just fyi.
EDIT: This may be wrong, but I am not sure...
EDIT: Some people think that you can fire an arrow from a carcaj and it will add the nine arrows to your inventory, minus the one you just shot. This is untested as of yet.
EDIT: Just tested it. The player will go through the animation of drawing back the bow, but when released, will not fire.