I would respectfully disagree with uncle matt. I have done a lot of work on my slug setup and rifled (Foster-style) slugs through a rifled barrel have not shown to be a worthwhile combination in my experience. I believe the manufacturers say the same thing. I would guess that the grooves in his barrel are significantly filled in/rounded. There is a different rate of twist between rifled barrel and rifled slug. The rifled barrel could likely shear off some of the rifling on the slug and who knows what you will get. I have shot Remington, Federal Classic (my favorite), Federal Premium, Winchester, Winchester Partition Gold and Hornaday through mine and never once have I had one tumble. My dad and uncle have the same setup as me with the same results. My setup is a Hastings Barrel, Beretta 390 and Burris scope and we are all consistently shooting sub 3" groups (outside to outside) at 100 yards off of a vise type rest. Do what other posters have said and try a box of each flavor and see what works best. Note, if you try the rifled slugs and they don't work, you are in for many hours of cleaning to get back to where you were before you shot them.