1. Pick which module (mod) you want to make the change to (you pick which module when you start the game), I'll pretend we are doing this for the default 'Native' module for the next steps.

2. Go to the mod folder, for Native: Steam\steamapps\common\MountBlade Warband\Modules\Native, and find the file item_kinds1.txt

3. Save a copy of item_kinds1.txt just in case, then you can do 1 of 2 things: Download the ItemEditor tool for the game, go to item 555 Hunting crossbow and change the accuracy, max ammo, and speed to 99999.

OR

Open item_kinds1.txt in a text editor, find the line that starts with "itm_hunting_crossbow" (no quotes), and replace that line (just the 1 line, the next 2 lines with 0s don't matter) with this: itm_hunting_crossbow Hunting_Crossbow Hunting_Crossbow 1  crossbow_a 0  6357001 5637160960 22 131082 2.250000 1 0 0 99999 0 0 99999 100 0 99999 293 0

4. If you start the game you can fire the hunting crossbow (which is the crossbow you start with, if you start with one) as fast as you can click. If you want to automatically click really fast, install AutoHotkey (if you've never used it, look up a quick tutorial, its simple to install and use), and use this code for your autohotkey script (i.e. put this into notepad, save as, set save as type to All files, and name it AutoClicker.ahk):

#MaxThreadsPerHotkey 3

XButton2::
Toggle := !Toggle
Loop
{
	If (!Toggle)
		Break
	Click
	Sleep 5 ; Make this number higher for slower clicks, lower for faster.
}

After you install autohotkey, and have that file saved as an .ahk just click it to start running it, XButton2 means the forward button on the mouse, but you can use any key combination you want.