This must be a very old inline if the breech plug does not come out. This isn't an Apollo is it?

You could get a flush kit which is a tube that hooks to a #11 nipple. You put that in place and then suck water through the tube. This can flush the barrel very well actually. People with fancy traditional rifles that have a pinned stock use them.

You can make one if you have a spare nipple. Take it to Wal Mart, go to the pet section and in the fish tank area, start checking tube size. There is one that will fit right over that #11 nipple. A little epoxy and you're in business. Remove the dirty one, put in your fish tank one, and again you have a flush kit for much less money.

Once you get the majority of the fowling out of the barrel, still run some solvent patches to make sure you are getting all of it out. And then some isopropyl alcohol patches to make sure the barrel is nice and dry. A few dry patches and you should be ready to oil.

Or, put the scope in a plastic bag, and dunk it in the tub with a sponge under the scope to soften the pressure to it, and pump the barrel out that way. Be sure to clean the tube, You can get in a lot of trouble for that if you leave fowling all over the tub, and it is nothing compared to the toilet seat trick for the better half....