I bought my cartridge holder from Cabela's. They are made by Boyt and come in 5 and 10 round. They are for short mags and ultra mags. I found that the short mag holder worked better for fast reloading. I use them with 470 and 500 ammo. Never had a round come out that I didn't want to, even crawling on the ground. I did put rivets on corners to be sure stitching didn't come lose.

Sam