Magnum 20 Gauge Buckshot loads are max of about 50 to 60 yards.

I would put it closer to 40 yards for the 20ga and about 45-50 for the 12ga because of the small number of the larger Buckshot in the 20 Ga. The 20 ga is better with the smaller sizes of Buck like number 3 buck which will have to rely on pattern density to be effective the 20 ga case is just to small to be effective with 00 or 000 buck

Slugs are good for max of 90 to 100 yards ethical hunting. A fully rifled barrel adds another 25 to 30 yards max.