Can you use one of the old style slugs in a rifled barrel, yes. BUT, shotgun slugs are made of very soft lead and as a result they will probably lead up your gun very quickly resulting in decreasing accuracy. I have seen this happen, fella was a cheapskate and would not pay for good quality 12 to 15 buck a box sabots and would only buy 3 buck a box slugs. At first, they shot great. But over time the accuracy decreased. Then he decided that it was time to shoot good quality sabots. Problem was that he never got the lead out and his accuracy still was bad. Sold the gun to a friend of mine shortly after and after some good scrubbing and brushing, now is a solid 2 inch shooter at 100 yards w/ sabots.
Every gun is different, but in general sabots are for rifling, fosters are for smooth.

Hank