5C Emergency Collets are soft and can be bored to size/depth/taper/shape to suit a particular component.
Pins or packing pieces are fitted to keep the collet 'open' during the boring operation which is carried out in a 5C lathe collet chuck.
Brass and nylon collets are used to prevent marring of delicate parts but being softer, do not grip as well as steel collets.
When no longer required, 5C emergency collets can be re-bored many times over to hold larger parts (so don't throw the pins away!).