My wife has had the NEF Tracker II in 20 gauge (the NEF version of your Ultra Slug) for 10 years. Never once had a misfire and she uses it every year. If yours has misfires, I would check the firing pin first and then shim the hammer spring. Why the firing pin first? I had a double O/U that I used for Sporting Clays and it beat up the firing pins with all the rounds thru the gun.
For ammo we have been using Federal Premium Slug Sabots in the 3" and 5/8 oz. Htdra-Shok HP Magnum variety. 2 MOA at 50 years and double that at 100 yards (but there has never even been a 50 yard shot while hunting, 40 yards being the longest. All one shot stops as well.