If the barrel has screw in chokes (Remchokes), 20" is fine. As kmon pointed out, a longer barrel helps with follow through, but a 20" barrel loses almost nothing else in performance. In a hunting gun I would prefer a barrel that is a couple inches short vs. a couple inches long. P_102
Do not trifle in the affairs of dragons, for thou art crunchy and taste good with ketchup.