EL34/6L6
Please re-read my post, #6, GREASE the bore with the Nylon brush, let the grease in the bore for some time, days at least, then patch it out at the range just before shooting it. The shooting is PART of the cleaning/de-rusting process. Grease again while barrel is still hot before casing it up for the return trip home. Grease gets into the pores of the metal and softens the crud and rust.
Swiss methology is to have grease protecting the bore at all times, beside actual shooting and more serious cleaning.
Previous custodian of your rifle did not adhere to that methology, and thereby allowed the rust to form.
The nylon brush is to coat the bore with the grease, not plug it up entirely, as like cosmoline.
Dip the brush into the grease or coat it from a grease tube. Pass it back and forth a few time to ensure even coat.
Prevents oxygen forming rust on steel and sulphur dioxide forming tarnish on copper.