Yes, I know you said the bores are clean- however, Remington did not use variable depth rifling, and Marlin used the shallow (Microgroove) rifling. It is RARE to find a non-antique 22 with a shot out bore. It IS common to find one with a lead fouled bore.
Find a product called KROIL- it is made by Kano labs, you can order on line- it is the BEST penetrant oil made. Sop the bore with KROIL, let set overnight, come back and brush, not uncommon to see chunks of crud fall out.