Alright! The smoke cleared out and I hit the backyard range with my Cricket II PLB (synthetic) and a Blackhound 6-24x50. Here are the slugs that I used and the groups I got with each at 15 yards:

JSB Match Diabolo Hades 26.54 grains (0.0") - this is my usual hunting pellet
Zan slug 30 grains (0.245")
Dale "D-Rig" Riggert (eBay) Varmint Knockers 45 grains (0.292")
Dale "D-Rig" Riggert (eBay) Varmint Knockers 34 grains (0.394")
FX Airguns Hybrid Slugs 26 grains (0.398")
Nielson Specialty Ammo slug 38.5 grains (0.411")
Nielson Specialty Ammo slug 29.5 grains (0.415")
Zan slug 33.00 grains (0.436")
Nielson Specialty Ammo slug 33.5 grains (0.451
Nielson Specialty Ammo slug 29 grains (0.478") (Flyer: 1.083")
Nielson Specialty Ammo slug 26.8 grains (0.775") (Flyer: 1.016")
Nielson Specialty Ammo slug 36.2 grains (0.857")