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.
What we have here is... failure- to communicate.