My nephew shoots nitro hevi mix 4x5x7 in a 870 21 inch barrel with an IC.555 and gets between 150 and 160 in a 10 @40. Have seen him smoke birds at 40 yards, I like Nitro shoot them in 20, 12, and 10 I shoot 4x5x7 mix in all mine. I have never had any trouble getting nitros to pattern. Cant say much about heavyweights never shot them went from lead to nitros about 8 years ago