When he was done, he could hit a 5 gal bucket at 275 yds 10/10 times
Are you sure about that? 275 yards? or did you mean feet? Using the
Hornady ballistics calculator and Remington's AccuTip bonded sabot slugs, zeroed at 150 yards, your round is going to be 50 inches (FOUR FEET) below your point of aim. That's a decent drop to be hitting it that accurately.
Of course, I'm guessing a ballistic coefficient of about .15 to get the 100yd rise numbers in the Hornady calculator to match Remington's ballistic chart. Also, your neighbor might have been using rounds with a flatter trajectory, but I don't know of many slugs that'll change my numbers that much.
Remington ballistic chart
Yes, 275 yds measured with a laser rangefinder. I printed out a ballistics table for him to use when hunting, that listed drop at various ranges.