Darren,
If you have a look at a 5c collet, if you thru bored at 30 mm, the back end would most probably drop off, as you would be taking away the strength of the collet.
If you need to go larger, in a home shop environment, you would need to go to home made split collets, used in your 3 jaw. But they aren't really for a production system, just one off jobs.
John