The boxes used are quite flimsy but are made more robust by an internal tray into which the bullets are arranged a bit like eggs in boxes. They can usually be opened at either end. Most of the weight is in the bullets themselves. When you open a box it is possible that you cannot distinguish if there are empty spaces at the other end. If you open one end and tip the box the tray and the rounds fall heavily in a heap and most of the rounds fall/dislodge from the internal tray.
SC's rampage started after June had retired for the night and it is likely that she talked to Pamela from the bedroom. She may not have seen the rounds on the work top or may have left them. They where not very house proud as can be seen in some of the crime scene photos.
The boxes are not flimsy at all. They're made of rigid injection-moulded plastic, of similar size in 1985 to what they are now, the only difference being the latter are all opaque black in one piece whereas the earlier versions had both a transparent base and an opaque top part clip-locked together. Both have a thin plastic lid (with stuck-on paper info label), which only needs sliding off to reveal the contents. With Bamber's box it was easy to see how many rounds were contained within, either through the transparent base or when the lid was completely removed as shown in the worktop c/s photo, where one round was left remaining in it, while the rest were tipped out making 30 in all (i.e. after loading the magazine at least twice to carry out the murders).
Bamber claimed in his first two w/stats that he retrieved an almost full single box, i.e. containing less than 50 rounds, and an empty magazine from the cupboard, tipped out the former on the kitchen worktop, then loaded the magazine with between 8-10 rounds.
There was no rampage by Sheila, and there was no ivory rotary-dial phone for June to use in the bedroom either; that was previously moved downstairs by one of the family to the worktop, and if Bamber was truthful about his magazine-filling episode, June must have known where he left the bullets and box because she was sitting behind him in the kitchen at the same time.