. . .the thing that is spaced particularly to control what length of cartridge you can put into the cylinder, stopping people from loading a .357 magnum round in a .38 special chamber for instance?
It's the chamber length point in a .38 Special revolver that's 1.1630" ahead of the breech face that stops longer cases (.357 Mag, for example) from being chambered. Ahead of that point is the cylinder's forcing cone tapering at a 6.5 degree angle to the chamber throat.
Ahead of the cylinder's the barrel with its own forcing cone
Check the following page from SAAMI:
http://www.saami.org/PubResources/CC...ecial%20+P.pdf
Yes, this is the thing that is either moved up or eliminated when a cylinder is reamed to accept longer cartridges.