I don't know how "respected" the "gunsmith" is you talked to, but shooting an 100% soft lead slug through your rifled slug barrel makes absolutely NO difference! I happen to be a professional trained gunsmith for 27+ years, and it sounds like your gunsmith must have been hitting the sauce or something. The ONLY thing I can POSSIBLY think of is after firing massive amounts of rifled slugs, there MAY be somewhat of a lead buildup in the bore, HIGHLY unlikely however, and easily solved with a cleaning.
ORIGINAL: Savage .17
You can shoot rifled slugs through a rifled barrel, it causes little to no damage at all to your barrel. However, I was told by a respected gunsmith that once you shoot a rifled slug through a rifled barrel, you must not switch back to sabots as this will cause damage to your barrel.