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I would generally recommend slugs over buckshot as well, but he said he wasn't shooting more than "20 to 30 yards". At that short distance I think buckshot will do fine. But further than that and I'd definitely want a slug instead.
If you're only shooting up to 30 yards, I'd probably use buckshot. Even #4 buck would work good at that range. You might want to buy a few different loads and pattern them to see which ones your gun likes best.