Most shotguns will prefer a couple of slug loads considerably more than others. Mine really shoots Brenneke KO's compared to others I have tried.
The first step is to hang a large target with multiple bullseyes at 50 yds after going to the store and picking up a box of several different brands of slug. Shoot the different brands of slugs for groups. Take your time and really concentrate on getting the best possible groups, not easy with the bone jarring recoil of a 12g. For my shoulder's sake I only shot 3 shot groups with each slug, the semi-auto may dampen recoil enough to try for 5 shot groups which would be preferred. Dont be concerned with with point of aim point of impact at this point. After you are done you will probably notice one or two of the slugs shot considerably better groups than the others. Pick the one that shot the best group. If two shot similar groups pick the one that is cheapest.
Go back to the store and stock up, preferably from the same lot number. The next time you hit the range shoot a 5 shop group. Adjust the sights to zero the group. Now you are good to go. This is how I did it when I deer hunted with slugs. I still have about 10 boxes of slugs that are sighted in for my gun and shoot decent groups.