For HD to everything else SHTF etc...
* Use a side saddle with Esstac Shotgun Card with velcro stick on the receiver (once the card goes empty you can rip it off ad replace it with another one rather than refilling the side saddle)
---
http://www.skdtac.com/Esstac-Shotgun-Card-p/ess.106.htm* 12ga Micro-Rig
---http://www.originalsoegear.com/collections/micro-rigs/products/12ga-micro-rig
* VTAC Shotgun Ammo Pouch (Holds one standard box of shotgun rounds)
---http://www.511tactical.com/shotgun-ammo-pouch-vtac.html
*California Competition Works Shell Caddy Tactical Shotshell Ammunition Carrier 12 Gauge 4 Round 2-3/4" and 3" Shells Polymer
---http://www.midwayusa.com/product/2394156343/california-competition-works-shell-caddy-tactical-shotshell-ammunition-carrier-12-gauge-4-round-2-3-4-and-3-shells-polymer
For competitions
*California Competition Works Shell Caddy Tactical Shotshell Ammunition Carrier 12 Gauge 4 Round 2-3/4" and 3" Shells Polymer
---http://www.midwayusa.com/product/2394156343/california-competition-works-shell-caddy-tactical-shotshell-ammunition-carrier-12-gauge-4-round-2-3-4-and-3-shells-polymer
* For the Load 2' carriers they are strictly for speed, indexing sacrificing ammo capacity
As for the bandoleer sling is good as to carry extra ammo but not good for reloads in a high stress situation, also weigh down the shotguns swing rate.