It sounds as if you have done
almost everything correctly. The one fly in the ointment is the use of Bore Butter to lubricate and protect your bore. It doesn't work for that. It's a good bullet/patch lubricant but has absolutely no rust preventative qualities. Your bore should be protected with something like Birchwood-Casey's Barricade. There are others and this is just one. Just make sure it has corrosion resistant properties and not just a lubricant.
As one person suggested, you can wrap 0000 steel wool around an old brass brush and scrub your bore to remove the last bit of rust. You can also use a piece of one of those green 3M Scotch Brite pads but you will have to use an undersized jag to get it down the bore.
The "flash rust" that some people talk about is a
very light surface rust that quickly forms in your bore (or other steel surface). Many claim that using hot water will cause it to form in your bore. I am not one of those people. I use hot soapy water to wash out my bore and I use hot water to rinse it. BUT....I don't let it sit around after rinsing it, I wipe it dry with dry patches and then spray WD-40 into the bore to displace any lingering moisture. Following that, I run several clean patches down the bore to remove the WD-40. It has little corrosion resistance so I remove it after it has displaced the residual moisture. You don't have to let it sit after spraying it with the WD-40 because the stuff works quickly to displace the moisture. Wipe out the WD-40 and then run a patch with Barricade on it through your bore and it will be in sweet shape to put away until your next outing with it. Of course, you need to be as attentive in cleaning the rest of your rifle.
If you have a caplock, be sure to remove the nipple before flushing the soapy water through the bore and then pay attention to cleaning the nipple. Also, pay attention to cleaning the nose recess of the hammer. Often spent caps will get caught in there along with the fouling from the firing of the caps and the black powder. I always remove my lock everytime. I have found that it is easily cleaned with automobile brake cleaner in a spray can. Just do not get the brake cleaner on your stock because it can damage your finish. With the lock out of your rifle, just spray it good with the brake cleaner to remove all fouling. It will quickly dry but I use compressed air to blow it dry. Then
lightly oil your lock and return it to your stock. Using too much oil to lubricate your lock will just give a place for fouling to gather and gum up your lock. I like to put a few drops on a Q-tip and wipe all of the inside surfces of the lock and then wipe off most of the oil with a rag. The light coating that is left is just right for protection from rust. For the moving parts, just the least drop is sufficient. Oh, I just happened to think....do not remove the so called "clean out" screw in the bolster if one is present. That hole is
not there for cleaning, it is where the manufacturer drilled out the flash channel and then used a screw to fill up the hole. Some manufacturers drill from the opposite side and then plug the hole so that it is not visable. Most, if not all, T/C rifles have this "clean out" hole. It doesn't hurt anything to remove this screw and run a pipecleaner through there but it is
not necessary and most folks simply ignore this little screw.
You mentioned the idea of "seasoning" your bore. That idea is one that was put out by the Bore Butter manufacturers some time back but who no longer tell you that you can season your bore with their product. It is
not correct and you cannot, nor do you need to, season a bore. Here is an article written by the late Paul Vallindigham, who was a friend of mine.
http://www.muzzleloadingforum.com/fusionbb/showtopic.php?tid/251958/pid/918147/post/last/m/1/#LAST