Throating reamer method?

[RevJim]

I admit I am not a gunsmith nor even close to one. However, when I read of guys having a chamber set up to shoot a certain bullet at so many thousands off the lands, how is that done? Does the barrel have to be taken out of the action, reamed, then head spaced back? Or does the throating reamer work through the action into the existing barreled action? If so, how does the smith keep the throat square/centered with the chamber? For example, if I have a short throat in a 7mm Rem Mag and I want to have an overall cartridge length of 3.5 inches with a 160 (from 3.25 say) how is that needed .25" accomplished? I'm curious as all get out!?