I have had good results with 27 to 27.5 grains of H322 and the 40 grain Vmax or Nosler BT bullet. If you have a 1-12" twist that should be just dandy for 40 grain slugs.
Even though a 50 or 55 grain bullet would be better for ranges past 300 yards or so, I have killed prairie dogs out past 400 yards with the 40 grain slug and it worked just fine. For ground squirrels the 40 grainer is just about perfect. I launched quite a few with that above load that I gave you.
Hope that it works for you.