Ammo Made Easy: Three Left on 3

Inspired by the cascading dice rule from Intwischa, but not really wanting to add another die roll to make it happen, I offer this way to track ammo in a d20 based game without tracking ammo, without rolling extra dice, and without the surrealism of "Surprise, you run out right now!"

• Assuming you start with a package of 16-20 ammo-bits, if you roll a 3 or 13 on the d20 to hit, you are down to 3 ammo-bits in that package until you can buy more ammo.
• You can't run out if you roll a 3 or 13 in the first three shots of your first encounter after stocking ammo.
• This assumes you're not retrieving missiles ... I guess it's not too much complication to say you run down only on a 3 if you retrieved missiles after the last battle, otherwise you run down on both numbers.

Together with some ideas for special missile weapon fumbles, I reckon this might justify one page in the Next 52. What do you think?