Regardless of the particular brand of shotgun, think in terms of the amount of constriction from your bore size. The Invector Plus nominal bore is .742". Somewhere around .050" constriction from your bore puts you in what is usually a sweet spot for No. Four Buck. A little math says that should be around .692". Since you are using 3.5" loads with a large payload of shot it might be prudent to open the constriction up some, so the .708" you have on hand might be a good starting point. If you don't like what you see on the pattern board (10" & 20" circles at point of aim at 40 yards) go tighter in .005" increments. Kick's, Indian Creek, Jebs and Carlson all make quality extended choke tubes. Just make sure you aren't using shotshells with a Flite Control type wad in a choke designed to strip or retard the wad from the shot column. That is self defeating and is a bad combination. Only time spent at the range patterning combinations of shells and chokes can settle the question of what is best for your gun. And don't be surprised if some loads don't impact at point of aim. Shotguns are a lot more finicky than most guys think.
