A 20" pump gun is about the same overall length as a 26" double. Maybe a little shorter. Properly choked no reason to think it wouldn't do as a hunting gun. Practice with it a bit to get used to the way it swings.
I'm a big fan of modified choke for buckshot, I've found them to USUALLY work fine, but I hunted with a lot of good hunters who used full choke guns for buckshot and they swore by them, too. As I said earlier, the only way to know for sure with your gun, and your load is to shoot it on paper.
You're not going to damage the choke shooting buckshot. Maybe if you were to use a really extra full turkey choke or something like that, but I've never heard of anyone damaging a normal full/modified/improved cylinder choke.
I agree that No 1 buck sounds perfect for your use, but then I think No 1 buck is about the perfect buckshot. But if I had 00, I'd use them for what you're talking about.
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