Good question. On my Rem 870, there are two tabs that retains shells in the magazine tube. I believe they are staggered, so that when the rear tab releases a shell (on the backstroke), the front tab is not retracted, so it catches the next shell in the sequence. Or something like that. I haven't paid too much attention to the process...it's an 870, so as long as I don't load a shell backwards, it's always going to work.