Side saddle for more ammo, buttstock ammo carrier (W/cheekpad is nice!), and an extended Mag tube. I also have the enlarged safety button which allows me to snip the safety off with the side of my trigger finger without changing my firing grip on the gun. That's all you need for a CSG.
The slings which carry ammo are no good. They're heavy with ammo on them and swing all over the place, no good.
And of course, the admonition of Dave McC...BA-UU-R.