What was to have stopped Jeremy saying," the gun's in the cupboard,dad ?" It would have been as good a hint as having left it where he did. So I don't buy that.
He made up the claim he left the gun and bullets out prior to the murders in an effort to suggest Sheila found a weapon of opportunity and the magazine was already loaded so Nevill would not have been able to catch her loading the magazine and to have stopped her before she could finish loading it and then inserting it in the gun. Furthermore, he claimed he loaded the magazine in front of her so police would believe she would know how to reload it. The gun was reloaded 2 different times.
Furthermore he claimed he left it out prior to the murders so that he could say the gun had no moderator of scope when he left it out. A crazy Sheila removing the scope would look very suspicious so he claimed it was already not attached. Furthermore he claimed the moderator wasn't attached so they would not know it had been used. He made up what he did for set reasons he wanted to accomplish with the lies.
He thought it less likely people would believe she went to the closet herself to target the gun and bullets out and if she did go to the closet herself the gun would have had the scope and moderator attached and she would not have removed them. His lies were to overcome these issues.