If your gun shoots a deadly pattern at 30 yards, don’t assume it will shoot an almost-as-deadly pattern at 35 or 40. Patterns don’t open at the same rate, for one thing. Open chokes tend to spread in a cone shape, opening steadily and falling apart quickly. Tight chokes often shoot patterns that stay tight for a while, the sort of “bloom,” opening up at longer ranges. In either case, patterns can decay very quickly after they open to an effective size. I’ve seen this with buckshot especially, but also with lots of different shells.