I was never quite clear why Jeremy purportedly told Julie he had to wipe the gun because a glove came off in the fight with Nevill. Maybe his fingerprints would be smudged in blood which necessitated towelling it down,but surely Sheila's prints wouldn't be on it at that stage?
It makes it sound like he told her that he planted Sheila's prints on the gun earlier by making her hold it but then during the struggle with Nevill they could have been wiped away by his gloved hands grabbing at the gun or Nevill's prints could have been added as he was grabbing at it or the killer's prints could have been added if a glove came off which might have happened. If we take her claim at face value he was vague enough to have implied MM's glove could have come off and resulted in him leaving a print. She doesn't say straight out he admitted he did it and tha this glove might have come off. Her claim still leaves th epossibility of Jeremy saying MM's glove might have came off.
Since he made up a lot of things he told her we have no way to know if any of this is true or not. I tend to believe he didn't make up a glove possibly coming off and that he was worried that happened but have some doubts about him planting Sheila's prints beforehand.
Still there were other accounts that right before the murders he was making Sheila handle a gun I forget which relative saw that and asked if it was wise. So you never know maybe he did make Sheila handle the gun to try to get her prints on it before carrying out the murders. He could have wiped it down and then gave it to her to handle and try to make sure she touched places where he would not ordinarily hold it. The accounts of him teaching her to use a gun could have been him trying to get her prints on it.
I don't remember how long before the murders that supposedly was though and worrying about fresh prints it would be best to try planting them after she was dead because blood could cover them and they could be wiped away from use.
We simply have no way to know for sure though what he did precisely to try to plant her prints. All we know is it didn't work well.