I made a javascript-based pretty little utility to automatically track ammunition but it's part of a whole digital character journal I built (195-page PDF). If you guys would like to try it out I can extract it from the journal and share it here.
Here's how it works: It's a PDF that can run on your tablet, phone or computer and there's a section where you add weapons and ammunition to your inventory via an "Add" button, entering all item details etc., and the item is populated in your inventory automatically. Then you go the equipment section where these items and their details are to be automatically generated once equipped, so you click a weapon - say, a bow - to be equipped, and then click a dropdown menu to choose what ammunition to load into the weapon, and then simply press "Shoot" to fire. With each press of the button, one unit of the ammunition is depleted from the inventory. Goodbye to ammunition counting.