This is the pattern I created for my Webley revolver holster. I do not have the means to scan it, so I took some pictures besides some rulers and over a marked grid.
https://imgur.com/a/SNthjwk
I usually make it in 10 ounce cowhide, sewn with 0.6 mm leather thread. I dye it with Fiebing's Dark Brown Oil Dye.
The end-cap has no pattern, I usually trace it after sewing the sides. You need to do a box stitch do make this work, but you can also sew the end flat, without a cap.
The outcome is like this:


Cheers,
Bear