Given the cost and limited size/availibity of 3C collets, you may be better off using a 5C collet vise such as Enco [
From time to time Enco has these collet vises on sale with a set of 5C collets.
Because 5C collets also fit other fixtures like a spin index Enco p/n
235-6011[29.95$US] and square/hex tumble blocks Enco p/n 235-7050 [34.95$US] this may be your most economical and flexible way to go.I have found this combination to be extremely useful and flexible for the types of projects I do.
When I need to work on a long straight rod/bar I have found it relatively quick to bolt a suitable v-block to the face plate. Key to this is making some sort of steady rest for the other end of the spindle to reduce tendency to whip and cantilever action.
When I work on something with a taper such as a barrel, I have found it helpful to make a clamp like a driver dog and bore the correct size and taper. Again you will need to rig a steady rest of some sort on the other end of the spindle. This will clamp the tapered item very firmly with no marring (use a single layer of aluminum foil between clamp and barrel) and with a little care can be set to very low or zero TIR.
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