I have been using the undertaker choke now for the past three years (this will be my forth season), and i have not been dissapointed in terms of missing a birds. Each year i have got my two birds and my longest shot was paced out at 43 yards (the bird dropped hard), and second longest shot being 40 yards. The key is to make sure you take your new choke out to pattern it, and when doing so, try a few different types of ammo (I prefer No.5's) and shoot at different intervals. 30, 35, 40 or whatever, but change it up, as shot and chokes pattern differently at different ranges, and you may find that a particular type of shot will not produce at certain distances. Also, i think a rule of thumb is to ensure you are having at least 5 pellets in the vitals on the target. If a bird is going to hang up a 40yards you need to be confident that you will take him down, and not wound him.
J