Rcaddel,
I shoot 3in slugs. I honestly dont need to use them for most of my hunting but i like to have the little extra power. A 2.75in slug has plenty of power and will be no problem in your gun. The 3in slugs have noticably more recoil then the 2.75in. Get a box of as many different slugs as you can and see what groups the best. Mine shoots best with 3in Federal Barnes Expander. The next 2 in the list which are only .1in biger groups at 50 yards are both 2.75in rounds. As for your nephew he did not mess the gun up. What happens is the soft lead in rifled slugs can let the slug turn sideways in the barrel and get it stuck. Ive put a few slugs through mine just to see how they do. They shoot good for rifled slugs but i have the rifled barrel and will never go back to rifled slugs through a smooth barrel.