I think you are thinking of the Remington factory rounds which used a .224 bullet incased in a sabot and loaded in a normal case. IIRC they were loaded in .30-06 & .308, maybe a couple of others. I never tried them, most reports I've read said they weren't terribly accurate.
"Jimmy, some of it's magic,
Some of it's tragic,
But I had a good life all the way."