I have found that Foster type slugs give the best oerformance from an improved clyinder or skeet choke. However,every barrel is slightly different and every ammo type is different so it is very important
to see hoe your gun will handle slugs.
Kart29 is quite correct in saying that some slugs are not intended for some barrels. Hastings warn not to shoot Remington Copper Solids in their barrels. I believe that some Dixie Slugs are also designed only for rifled barrels. Of course any older European barrel that is marked "NOT FOR BALL" should not be used with solid ball ammunition.