Ferrules are very critical for preserving the ends of compressionless housing, and must be sized for a slop-free fit to prevent any of the lengthwise strands from migrating into the hole that the cable must pass through.

I sometimes have to search to find the right-sized ferrule for both the housing and for the cable stop. I have taken to keeping my ferrules more organized since I use a lot of them and hate to waste time searching for the right one.

The particular ID size of some late-'80's Schwinn's housing stops under the chainstay has recently given me some grief, had to spread them open to get a ~5mm stepped ferrule to fit. Seems they used standard housing with no ferrule on the 6s models, but that had to go for good shifting to be realized.