Where is there proof that Jeremy ever left the gun where he said he did. His cover story needed it to be somewhere in which Sheila's deranged eyes would fall on it, but it's unlikely that any of the three remaining adults would have left a gun laying around. If Jeremy left it anywhere, I suspect it would have been in a place where he could easily lay hands on it later, which may -or not- have been the gun cupboard.
It is possible Jeremy left the gun out ...
I would be shocked if Neville had not put it away in the gun cupboard ...
Only because his grandchildren were in the house ...
But if this theory is correct then the person or people would have known they would find an anshuz and ammunition in the gun cupboard ...
Anthony was there days before paying great attention to were the 2 anshuz rifles and ammunition were ...
Read his own statement about it - it's in the forum ...
He claims that he took his own anshuz rifle away on that occasion ...
Coincidence - maybe ...
Maybe not ...