Blazin,
Thank you for the compliments. They are really pretty simple to make. It takes me more time to stamp the border than anything else. For the pattern I have one basic pattern I build all of them off of when I start a new rifle. Basically sit a piece of paper under the rifles stock and trace it. This is your pattern for both sides plus a little in the middle.
The loops are all one pieces, same as sewn ammo loops on a cowboy belt. I sew them on a machine so It's up one side then under the loop across, then back across the strap. I measure before hand using the width of the cartridge + + two times the width of the leather (each side of the loop. You'll also need to determine the width each loop will approximately take up which I do my simply mocking up the loop and taking measurements. This will tell you how many loops you can get in a given area.
I included a flat photo to help in the visual. What I'm learned from early pieces is that it's better to have the pieces shorter than longer. It will stretch over time so you want to be able to keep tightening the laces.
-Robert