Hi Doug, welcome to GBO!
No info on the misfires for you but I'm sure someone with a clue will be along shortly.
One thought on your range session. It might have been too long. 40 shots from a 12 ga slugger is a pounding. Both on the shooter and equipment. Check your scope for tightness and your shoulder as well...
By the way, what was the difference between the groups at 50 as opposed to 100 yards?
Hope the experts show up soon to address your questions.
Nixter
Ps. Local shop has an Ultra Slugger Deluxe that I've had my eye on, going to check on it tomorrow....