Scab board took me a while... but the word you're looking for is scabbard. You really don't need a bulky shotgun strapped to your back if you're sniping. A pistol IS heavier, but heavy is a relative term. You don't notice a big honking shotty on your back, but you think a pistol will be more of a burden? Just get a good holster and you'll forget it's there. And a spring shotgun isn't a great backup weapon for a spring sniper rifle. You generally want to fall back to something that will allow you to increase your rate of fire when people close in.
And just get a belt to carry your pouches. You'll have the same problems with a plate carrier that you do with a chest rig.
Not for glory, nor riches, nor honours, but for freedom alone, which no good man gives up except with his life.