To find this .50 cal ammo can, you will have to have read the story. It's just under 3900 pages altogether. Why not start now?
There is one number that holds a predominant position above all others. Simply subtract that number from the last three digits of latitude and longitude to come up with the correct coordinates.
So do the math, or leave it to Ka.
Books of the series are:
1-The Gunslinger
2-The Drawing of the Three
3-The Wastelands
4-Wizard and Glass
5-Wolves of the Calla
6-Song of Susannah
7-The Dark Tower
ba orvat...