With respect to the Remington detachable magazine system - the machining on the bottom of the receiver is specific for that model. I retrofitted a Remington stock and magazine system to a standard ADL/BDL receiver, but it took a milling machine.
The aftermarket systems generally use a single position feed magazine, to get around the need to alter the receiver. On my competition rifle, I installed a PT&G unit (from juanvaldez), which uses AICS pattern magazines. This unit requires significant inletting of the stock, and installation of pillars. Other units, the HS being one, fit BDL inletting. HS has changed their magazine design; new pattern don't work with old pattern units.
From a shooting standpoint, adding a magazine system doesn't make the rifle shoot more accurately, but it makes sense for certain shooting situations. Ease of reloading is important in competitions where the exposure of targets is controlled in the butts, and they don't care if you are fumbling rounds into an internal magazine.
One of the better buys in a replacement stock is to get a take-off HS stock. Guys buy the rifle, then install the barrelled action is a chassis or other more expensive unit; sell the HS to recover some of the cost. These usually get listed for $250-$300, and are good value.
