Nikon D300 When I depress the shutter half way or

Only reason the lens will not auto focus when you depress the shutter half way is due to either lens AF switch is set to Manual OR camera body's focus system is on M. There is 3 way toggle switch below the synch port around the lens mount. It should be set to C or S in order for auto focus to work. If all the settings are correct and even "AF-ON", next to rear dial doesn't activate auto focus then the issue maybe in the setting or the body itself. As for the buffer reading r27, I think mine displays r## regardless of burst/continuous mode.