I think the SKS is a great SHTF gun, but a 12 guage shotgun might be a better option.

1)Ammo is cheap and plentyful for a 12 guage.
2)You can buy 12 guage shell almost everywhere.
3)You can get a variety of shells like, slugs, birdshot and buckshot.
5)You can use a 12 guage for home defense and for hunting a variet of game from birds to larger game like deer.
6)very reliable, pump shotguns don't jam very often.

The only downfall I can think of from a shotgun is you don't have the distance you have from a rifle.

Here is a Mossburg 500 Cruiser, which is a great SHTF gun.