If he'd wiped the rifle down there wouldn't have been any of his prints, let alone one.?
I think there could (and probably would) still be some of his prints. For me, the issue is more that, once he has wiped the rifle down, why do we not find more of Sheila's prints there? After all, if he is the killer, he is supposed to be staging this to make it look like Sheila has handled the rifle.
In other words, imagining Jeremy as the killer:
1. He wears gloves but notices his blooded gloved prints on the rifle.
2. A glove also comes off, which means he may have touched the rifle.
3. Notwithstanding he has been using the rifle lawfully anyway, due to 1 and 2 above he decides to wipe the rifle.
4. He then attempts to put Sheila's prints on the rifle after killing her.
In these circumstances, how come he only manages to leave one print of Sheila's, on the stock? If you follow the chronology above, it doesn't make sense to me, even allowing for possible difficulties with leaving prints.
On the other hand, if we say Sheila is the killer, then she may have wiped the rifle as part of her ritual washing, and with her hands and fingers are clean and dry, she may not leave any further discernible prints, even if handling the rifle in order to kill herself.