Not only the oils from the hands, but the oils on the rails and in and around the firearm.
I had a 9mm I kept loaded. On rotating out ammo, the old stuff was taken to the range and shot.
The hangfires I got reminded me of shooting black powder, but I was shooting a High-Power clone.
The bullet setback as shown above can happen on the first or 15th time of loading, but it does happen.
"A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."
Life Member NRA
Life Member NAHC