Here is the information that I've copied from several sources. Hopefully it's correct.
Pandora style use a 3/16th mandrel if you plan on coring them. If you don't, then make them on a 5/32.
Pandora chains are 4.2mm
Biagi chains are 4.1mm
Troll chains are less than 4mm
European chains, the minimum hole size is 4.37mm
Uncored beads made on 5/32" mandrels will fit because 5/32" is 0.15625" which is 3.96875mm provided that your bead release is 0.2mm thick
If you core your beads and want to use 3/16" mandrels, you must get the special sterling silver tubings Debbi Hackeson/ginkodesigns worked with Indian Jewelry Supply to custom produce the ONLY 5.08mm tubings that will fit Pandora chains allowing you to use 3/16" mandrels. 27 gauge, 5.08mm OD, 4.37mm ID are the important specifications. The standard 3/16" tubings from Rio Grande and other suppliers will NOT work.
http://www.etsy.com/shop/ginkodesigns
Even then, the mandrel is 3/16" which is 4.7625mm. The tubing is a smidge over 5mm. Thus if you have a thin coat of bead release, you do need to dremel the hole out a bit to fit.
Like I said this is just accumulated information. I've never done any coring. Hopefully it's correct and helpful.