Originally Posted by
KCT1986
If you are using common 4mm housing, then it will generally require an end cap. In addition to supporting the end and outside of the housing, the end cap will center the housing and keep it aligned properly.
If the housing end sits at an angle, the supporting strand can migrate into the opening for the inner cable (which is usually much larger than standard shifter cable). See the effect below.
In this picture you have a 4mm shifter housing going into a ferrule for 5mm (brake) housing. Of course it works poorly.
In reference to other advice offered here, don't try sticking a ferrule in a Shimano brifter brake channel. It will wedge in there and be really difficult to get out. If the brake gets used that way it can crack the brake lever body.
Shimano road brakes don't use ferrules at the caliper or lever. I don't know of any index shift system that doesn't use ferrules at both ends.