So if Bamber had used the rifle earlier in the evening why would he have bothered to wipe them off? Answering whether Sheila's hands were sweaty or not, it matters not because it is generally not the sweat that leaves fingerprints, but the oil in the skin and may I suggest that the rifle and the butt were both shiny enough for any prints to stick whether the person be dead or no?
1) Sweat leaves latent prints. It is 1 of 2 natural body functions that result in leaving prints the other is oil the body forms to cleanse the skin
2) foreign substances on the hands OR on the object being touched enable prints to be left. The ifle had blood on it, touching such blood coudl result in leaving a print on the gun and transferring blood to the fingers so that the fingers could leave bloody prints on other objects as well.
3) Prior to committing the murders Jeremy likely wiped the gun down to try to eliminate his prints from having handled the gun in the past. That is the most rational explanation for so few prints being found on the gun including none at all from Nevill. Wiping it down after would have washed the blood away so it wasn't washed after.
4) If Jeremy was the killer then the tale about taking the gun out to shoot rabbits is pure fiction. He took out the gun to take off the scope and load the magazine so that later on he could just take it and use it. No way would he leave the gun around where someone could end up hiding it so he could not find it. He either would put it back in the closet or hide it someplace where only he would know to go fetch it. He could have wiped it down after he removed the scope or right before he carried out the murders we have no way to know which.
6) A dead person with clean hands will be more difficult to get to leave fingerprints than someone with dirty hands. The cleaner the object like the stock which was not full of oil or grease or the like the harder to get to leave prints. A dead person also will have less oil on their fingers than someone living because the skin stops secreting oil so the sooner the better and your best bet is to take a finger like you are fingerprinting the person abd press each finger hard and roll it. It is highly doubtful Jeremy did that or there would have likely been more prints successfully left.
7) criminals don't think about everything in advance that they should, don't know all there is to know about staging scenes and planting evidence and make mistakes. The news just had a guy who broke into a house and got busted because he broke in and during the course of the burglary decided to check his facebook page and forgot to sign out of his facebook account and left the computer still on.