Currently it seems that 'r' only allows you to reload without needing to enter your inventory if you have your ammo loaded into a magazine, speed-loader, snap-loader, clip.

Pressing 'r' should let any weapon begin loading its ammo as long as the rounds are loose/not in a box. For example, if using a break action shotgun you should be able to press 'r' and break open the gun and place new shells in the gun. Currently the advantage of magazines and the like, are that they allow the use of 'r', but the advantage that they SHOULD offer is that the load faster.

For example a break action shotgun WITH a snap-loader should load twice as fast as one without, but both options should still be capable of using 'r' to reload.