Best advice is..... DON'T.
The sling full of shells swinging around make controlling the gun nearly impossible.
You swing the gun on target and attempt to shoot, and the inertia of the shells swinging around cause the gun to be pulled off target.
If you want more on-board ammo, you might look at a Speedfeed magazine stock which holds 4 rounds inside the stock in spring loaded "magazines". You can reach back and flip extra rounds out.
Also there's the butt cuff type which is an elastic band that fits around the stock and holds more ammo.
Just remember, the more ammo you carry on-board, the slower the gun will be in action.
If you're likely to be facing a enraged biker gang you may need the ammo. If you're more likely to be facing one or two intruders, the extra ammo will actually be a handicap.