RE: Deer Gun for 8 year old
You can put a limb saver recoil pad on the shotgun for about $25-30, and it will reduce the recoil to pretty much nothing. However, I don't know how accurate he can be at 100 yards with a foster slug and bead sight. You may want to buy a rifled barrel.
If you can use a rifle, any of these should be great: 357mag, 30-30, 243, 257, 25-06, 260, 7mm08, 308. If recoil is too stout on any of these (most likely not), put the limb saver recoil pad on it.
I put a slip-on limb saver recoil pad on my 12ga Nova, and it makes 3 1/2" high velocity buckshot feel like a 35Rem, which is a substantial reduction in pain. I can shoot 100 shells without flinching or bruising.