if it is in a plastic shot cup it should have no affect on the barrel but may not pattern well either. i am assuming you are speaking of a rifled barrel.
Brent
As has been mentioned earlier, it probably wont hurt the gun but it will hurt your "pattern" (in this case, I use the term loosely).
Save the slug barrel for its intended purpose...firing slugs. If you want to shoot buckshot, go out and buy a nice smoothbore or IC barrel (based on your model and needs).
Might cause a lot of leading in a rifled barrel, shouldn't do any permanent damage tho. But the results might not resemble what anyone would call "a pattern."
The slug barrel I currently have is a smooth bore barrel intended for riffled slugs. thanks for the replys so far. the box of truth link had some usefull onfo although not quite what I was asking. I guess I should have been more clear with my question. the barrel I have is a smoothbore.
If it's not rifled theres no problem. If it's a smoothbore it will do fine. I suspect it's probably just a barrel with a cylinder bore or I/C choke which are commonly recommended for shooting rifled slugs but are also used for shot.