Hi Cherrie,
When I started coring about 2 yrs. ago, I made a list of resources that had good tips & info. I'll share it with ya:
A good discussion on coring
http://lampworketc.com/forums/showthread.php?t=4386
Youtube video on how to prepare your metal tube
http://www.youtube.com/watch?v=_d_-d...eature=related
Tubing discussion, including sizes
http://lampworketc.com/forums/showth...=silver+tubing
Hole size & mandrel size discussion
http://lampworketc.com/forums/showth...andora+mandrel
When I first started coring (and realized how expensive a foot of silver tubing is) I decided to practice using copper & brass. I went to the hardware store with a couple beads in my pocket and used them to measure what size tubing I needed. Of course, silver feels a bit different from copper or brass when you core it, but there are plenty of skills a new corer needs to practice that are applicable for cheap brass & copper instead of using your expensive silver. I know Ginko on Etsy also sells aluminum, so you could practice on that, too. Likewise, you'll find some at your local hardware store, usually in with the copper & brass.