Welcome to the Forum.
I'm not sure what cleaner your using but if it is made for modern smokeless powder guns it will not dissolve the fouling and the salts that are in it.
The black powder bore solvents or cleaners are about 98+ percent water and expensive.
I've had very bad luck with TC #13 black powder solvent causing almost instant surface rust in the bore so I do not recommend it.
Dish washing soap with water does an excellent job.
So do some of the windshield washer fluids although Windex has ammonia in it and that can cause rust to form.
As for the need to remove the barrel, no it isn't manditory however because it is so easy to remove from most of the halfstock rifles with a hooked breech I see no reason not to do it.
Your gun very likely has a chambered breech on it.
That means there is a chamber in the breech that is smaller than the bore diameter.
Intersecting this chamber is an angular hole that connects the rear of the chamber with the hole the nipple screws into.
These little holes also need to be cleaned and one of the easiest ways to do it is to remove the barrel and the nipple.
Place the breech into a bucket of water.
Apply some soap and water to a patch and run it down the bore.
Then pull it back almost to the muzzle.
This will pump water thru the smaller holes into the bore.
Pushing down on the cleaning rod will forcefully blow water back thru the small holes and blow away any fouling that has accumulated there.
Do this several times. Then repeat with a clean wet patch.
Run dry patches down the bore to dry it and then lubricate the bore with a good rust preventative gun oil. I like Birchwood Casey Barricade.
The Bore Butter and similar lubes are meant for lubricating the patched ball or grooved bullet.
Although TC likes to say this is good for rust prevention we have found that it is actually rather poor.
As the others have mentioned, there are a number of posts about cleaning a muzzleloader with lots of suggestions of what our members have found to work well for them.
Again, Welcome to our site.