Hi, O_S. Welcome to Mossberg Owners from E TN. We are practically neighbors.
I used to shoot 3-gun in the late '90s and early 2000s in Atlanta, actually at South River near Conyers, GA.
I modified my M500A with a Knoxx Sidewinder Conversion Kit w/ 10 round drum and 6 round box magazines.
Back in 2003 the kit cost $190 for early purchasers. Included forearm and grip, mag receiver, mounting hardware, and one 10-round drum mag. We early purchasers got a free 6-round box mag. Cost of extra mags was $90 for drums and $50 for boxes. I bought one of each.
I found the best way to manage the ammo was load the 10-round drum for the first shots and reload if needed with the 6-round boxes. Only one mag change was need because the stages didn't require more than 17 rounds out of a SG. I carried the box mags in an old WWII gas mask case.