Ranting is correct...
Once you learn what "plane" on the rear to line up with the bead, you are basically close to hitting what you aim at with slugs out to 65-100 (my ability) yards. For deer hunting, I personally refrain from shooting to 100 yards unless my family has empty bellies. No deer is safe at 65 to 80 yards.
What he says of practice is paramount. My mossberg (like a rem 870) has a 1/4 to 3/8ths inch flat top on the receiver. Getting that and the bead aligned is what you are after.
Brent