My biggest slug success has also been with the .177 NSA 12.5s. The gun is a Brocock Concept Elite in my case. I messed with the hammer length to soup it up a bit. It throws them at 880-890 for roughly 22fpe.
I can hit 2 inch spinners at 125 and 140 yards pretty regularly with it. Took a Euro dove off a fence at 67 yards last week. Been too busy this summer but last summer I was shooting pdogs with it from 30 to out past 100 yards. It'd kill em, but not quite as cleanly as a roughly 20fpe pellet at similar distance. Seemed to ice-pick them a bit. Still very cool to grab the gun on a day where the wind's really howling and be able to swing those long range paddles with minimal hold off (relative to pellets at similar fpe).