When making a finish reamer, what rules apply?....is it made exactly the same dimensions as what you want the case to be?
I have a 270 RCBS reamer that I bought from a forum member and I want to have it re-ground to give a shorter neck and some other changes.

Roger

They are made 2-3 thousandths larger than the brass; reamer makers will take a sized piece of your brass and go from there, That way they know the resulting chamber will be compatible with your dies.
But for a re grind, they will move everything back.
Your question is for the reamer maker to answer. However, if you alter that brass to a different dimension than your dies..... Neck length alone won't matter except for crimping.
And then what you will use for a sizing die? Custom die? Those cost $.
But for production reamers, makers all have their own dimensions, but most all fall within the SAAMI specs, so that dies, reamers, ammo, and brass, all work. Mostly.

Yuma,

As DPCD said chamber is .002-.003 bigger than brass at base of cartridge and shoulder, and .004 greater than neck diameter. Which means the die's neck had to be cut .007 smaller to account for .002-.003 springback assuming you're sizing for .002 neck tension.

Neck length should be at least .9 x caliber for best accuracy.

This is all per Wildcatting class I took from Fred Zeglin at Trinidad. He got the relative sizing info from Dave Manson. Dave could help you. I also highly recommend Fred's book "Taming the Wildcat". Lots of good cartridge design info.

HTH