That's a probability argument. Touching a small area like the trigger or a shell probably would leave a smudged print, if any. The finger leaving off the trigger would "wipe" any print there, as would pushing a shell forward between fingers into a revolver. There's also the factor that fingers don't constantly and consistently cast off prints anyway. Sometimes the fingers are dry due to touching something absorbent or purposely wiping or washing them.
Someone needs to load a S&W from scratch and test your hypothesis. Oswald must have only touched the rim of the casings not to leave even a smeared print on at least one of them. Otherwise, Oswald would have taken great care not to put his prints on them or the gun.
Police seldom see latent fingerprints on a spent shell due to heat vaporizing the oils. We went over on the Forum a while ago the new technique that would be able to detect "micro-etched fingerprints", prints that aren't visible but have left a thin residual mark due to heat. But some studies intentionally placed strong fingerprints on the hulls before firing, which improved the detection results. Still on the off-chance there might be some prints on the hulls in the JFK case, then they'll hopefully be tested when the science is truly ready.
If there was any hint of prints on the hulls, smeared or otherwise, we would have heard about it by now.
Some materials resist printing. I think the rifle's wood stock was like that, in that it was absorbent. The triggerguard housing was smooth metal, almost perfect for a print to be deposited. Without getting into the arguments for-and-against, some believe Oswald's prints were photographed on the housing.
Oswald's alleged palm print was discovered on the barrel under the stock by Day after he disassembled the rifle. But I'm still waiting to see the expert analysis that matches this print to Oswald. Supposedly, there were several detectives/laymen who eyeballed the faded print and declared it a match to the palm print taken from Oswald post-mortem. Those were some eagle-eyed cops to pull that off.
Fingerprint evidence is always highlighted on TV shows but real-life court cases usually don't have much, if any, in the way of fingerprint evidence. Often the weapon is wiped down or the prints are smudged or the surface of the weapon isn't receptive to prints. Or the killer wore gloves. The Golden State Killer apparently left no prints, only one trace of DNA.
But nobody thinks that Oswald wore gloves and he allegedly man-handled the MC by disassembling it, placing it in a paper bag (wonky scope included), reassembled it in the TSBD, fired it at least 3 times, ditched it and only left one questionable partial palm print on the barrel and no other prints on the stock, bolt, scope, strap, clip and ammo. The most likely scenario to leave virtually no prints on the MC, was that the shooter wore gloves. Otherwise, there is no way Oswald would have or could have wiped the MC down before fleeing the scene.
