I agree with Birman,that is the most likely cause,A liquid cannot be compressed so as the bullet passes over it the liquid either deforms the bullet,is forced into the metal of the barrel or a combination of both.

I have seen a blister like bulge a quarter of an inch high & over half an inch in diameter form on the outside of a friends rifle barrel after he fired it without running a dry patch through it first,he always had it heavily covered with oil.

I always run at least 1 dry patch through the bore before i take a rifle out shooting.