I use the 405 grain Lee flat nose bullet(457-405-F), pushed to about 1300 fps with a middling load of 4895 and a pinch of dacron to hold the powder against the primer. I cast them from dead-soft lead, because I'm shooting them at BP velocities. My Handi-rifle has a microgroove barrel, from the time when Marlin was making barrels for H&R and that load shoots just fine in my barrel.
Is your Guide Gun microgrooved, or Ballard rifled? Some folks report having trouble getting microgroove barrels to shoot cast bullets, but I shoot them as-cast and they do just fine.