I know there is a difference but, ignoring the obvious, what is it?

I'm dense enough that an example using both reamer dimensions and case dimensions for a typical cartridge would make sure it was clear for me.

In many cases the difference between SAAMI specs for brass and chambers are so different it's hard to see the relationship! The one thing that is plain is the minimum size for the chamber must necessarily be larger than the maximum size for the brass. It seems that often this is taken to ridiculous extremes however. Add to this the fact that you seldom see brass which is anywhere near maximum and you can see why specifying dimensions for reamers can be such a challenge for gunsmiths. On one hand it makes perfect sense to specify a minimum spec SAAMI dimensioned reamer. On the other hand, when combined with brass which may be at the extreme lower end of the spec sheet, this may yield a chamber which might be perceived to be way too big.
With the 30/06 as an example, typical case size will range from a diameter of .463 at the base up to, rarely, .468. Chambers will typically go .471 up to .473. The neck diameter of a loaded cartridge will range from .332 up to .336. Chambers usually around .340. The reamer will usually produce a chamber which is about .015 longer than the nominal case length.
As I said, trying to make sure reamer size is suited to the brass without being too tight for reliability can be a maddening exercise. If the gunsmith works within industry standards he may still be accused of producing a "sloppy" chamber simply because the brass is so small. If he tailors the reamer to suit a particular lot of brass he risks producing chambers which will be unusable with other brands. Also this reamer may be too small to clean up a factory chamber in a "set back and rechamber" scenario. Things are tough! Regards, Bill.

Is there a site anywhere where you can go and look a reamer drawings?

Try Clymers

Bill, take a look here at this site.

[ 08-26-2003, 07:26: Message edited by: Customstox ]

Bill Leeper said it all. Generally I try to make all my case dimensions about .003 inches smaller than the actual chamber dimensions. Of course one needs to consider this in having a reamer made and then there is the reloading Die dimensions. If you spend a bit of time thinking about it, it works out pretty well.-Rob