I bought a used FL 6mm Rem die off ebay for cheap. Everything was good until i went to size up some cases, all of which the bottom half inch of the brass did not want to go in. Well after forcing one in, i get the dreaded stuck case. I have never had this problem before with any of my other dies. The die body says RCBS 6mm Rem FL S. Does this S stand for sizing, or small base?

'S' would mean sizing die.

I believe "small base" sizing dies are marked 'SB'.

It sounds like you didn't adequately lube your case.

George

the chamber may be on the high size a fl die has to size the whole case. like stated try a little more lube.
Dave