I bought my Challenger because I had a NOE pellet mold for .30, and needed a gun to shoot them, also have some NSA slugs that fit the mag and shoot well, main thing is they need to be heavy enough to stay sub sonic, about a 50 gr pellet, and 60 gr slug will fit the mag and do quite well also.
My bad I got the maximum weight that would fit the mag mixed up with the actual ammo I have. I have the 54.5 gr. IMO they are possibly slightly undersize, feed real easy in the breach, but shoot well.https://nielsenspecialtyammo.com/collections/30-cal/products/54-5-grain-175-count-300-slug-nsa