The whole argument is based on an assumption that Sheila would notice the bullets and later remember them being there prior to first having to reload.
The defence narrative is simple. After shooting eight bullets into June and Nevill she then goes to where she knows the ammo is kept and loads five bullets. Realising soon after there is another source of ammunition on the kitchen worktop.
A prosecutions narrative is actually more problematic. Since only five bullets were taken from the cupboard and 20 from the kitchen table. Hence the killer used 20 bullets from the kitchen. Why then would Jeremy take five from the cupboard? He would undoubtedly know about the ones on the kitchen table since he put them there.
According to DB the shells were at the back of the cupboard in a carrier bag, 5 full boxes and one half full. According to your scenario, Sheila first of all must have known where the shells were kept, opened the carrier bag and took ONLY five shells. She must have taken them from the open pack and placed the pack back in the carrier - then off she went on the rampage again (have a break moment?). An altogether unlikely scenario the say the least!
What makes more sense (and isn't problematic at all - far from it). When Jeremy loaded the rifle, the magazine clearly already had 5 bullets loaded! He simply filled it with the rest from the new pack. He didn't mention that there were already bullets loaded because police would likely have asked why he them took time to load more, given that he was 'in a hurry' to shoot the rabbits, he couldn't say that he shot the rabbits either because, where are they? If they escaped - where are the spent bullets and casings.
He made a bit of a boob and one the police SHOULD have pursued.
