With a custom chambering reamer is made you can specify neck dia. the reamer cuts to. Normally this is a dimension that gives .001-.002 clearence on factory loaded ammo or sammi spec ammo. This is the no turn neck you refer to, the standard factory type chamber. On a custom reamer for benchrest or serious match shootig a Tight neck or turned neck throat is specified this is .005-.007 smaller than the loaded sammi diameter of the neck for the given cartridge, meaning necks have to be turned before being oaded to allow .0005-.001 clearance in the chamber. This is a means of keeping the loaded round on center and true to the bore of the rifle as possible. Brass done for a tight neck is normally dedicated to that rifle. Some benchrest shooters fit to chamber is such that there is no need to size cases as spring back is more than clearence is. Its a a way of gaining that last bit of accuracy available.