I'm probably ignorant about the subject, but seems to me a thin film of oil in a barrel will have less resistance than one that is bare. That first shot scrapes out the oil, and subsequent shots will hit at a different spot.
"Fouling shot", am I right?
Diameter of the bullet is large enough to permit the lands to engrave the bullet. Oil is not a lubricant in that scenario but an obstruction that must be removed. A high viscosity oil used for lubrication and metal preservation creates a hydraulic action that impedes the first bullet coming out of the oil coated barrel. The faster the bullet the more it is affected.
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