Just out of curiosity, have you fired it yet? You didn't mention whether you had or not. Before you do anything to the gas ports, fire the weapon with various ammunition types (buck, slug, birdshot) to see if it's even necessary. If it isn't, you'll save save yourself a lot of time and aggravation, not to mention avoid possibly damaging your S-12. A thought...
Yes I've tried a various mixture of 2 3/4 and 3 inch shells. All the slugs, 00 buck, and higher brass loads cycled flawlessly. The gun has been broken in and still have a pretty big inconsistency with the cheap wally world ammo.