[Yo-Yo]

Look at this video of a cutaway RFB, and pay particular attention to the spent brass at 0:31 in:

http://www.youtube.com/watch?v=DLO7rHjHerk

You'll notice that as the last round is fired and the bolt locks open, one empty brass case slides back, hits the stop, then slides forward a bit again. That prevents what you're afraid of.

[B4Ranch]

Thanks for digging out the video.