I keep a mesa shell holder on 1 of 4 Mossbergs that I have just to remind me why the other 3 don't have a ammo holder .
1)dump pouch for carrying the easiest ,most ammo , something like 50-100rds and the cheapest.
2)Micro Rig , chest rig or vest for the grab and go , ready ammo where you need it , close to the gun , i.e. chest level , almost as close as a side saddle is , and at least 25rd.
3)California comp works strippers or the like , I've got a couple , these work great , come in 4rd , 6rd and 4x4rd,so not as many reloads as the above,but you can strip off 4-6 rds at a time for your speed reloads.
4) bandolier ,.....lots of ammo....don't like running with one on.....?
5) shorts....Only enough room in my shorts for my mag lite.