Many 5C collets have an internal thread at the back end. This is to install a stop provided your workpiece is shorter that the collet. You can buy the stops or you could thread up your own as an exercise in thread cutting.
If your collets do not have the internal threads either buy new ones or simply make a plug and pin it in place.
In both cases of course the threaded or pinned stop insert has to be apped for a length of threaded rod to use as the actual adjustable stop.
An open mind is a virtue...so long as all the common sense has not leaked out.