Thank you John for changing access to that site as requested. 
Thank you, that was kind of you. It really put me off my tea.
I would inclined to think the magazine would not be in place when the rifle was used as a club because they are not locked in place, they are held in place with springs are they not? I must admit I am no gun expert. So if you are hitting someone with a rifle the magazine would be prone to fly out? Perhaps? Also if the gunman was out of ammunition at the time he would be more inclined to use the rifle as a club rather than in the usual way at least until he had disabled his victim sufficiently to get time to reload.
So I suggest a sequence: Empty the magazine into the first victims, run out of ammunition. Realize Mr Bamber was escaping, use the rifle as a club, break the stock, reload with single bullets to shoot Mr Bamber, locate the magazine and reload fully, go back upstairs to complete the massace.